'When I cry on screen, those are real tears - that's how I cry.

Twenty minutes into my interview with actor John Simm, it feels over already. Every question stalled. "I just don’t like anybody knowing anything about me personally," he says, eyes flickering down into his drink. "I find it intrusive." Why be an actor, then? Go and be an anonymous dustbin man and give us all peace. "Interviews..." he says gloomily. "I always end up sounding like this grumpy arsehole when I read them back." I try feigning surprise but I’m not as good an actor as he is.

Why do interviews then? Because he has to, he says. He needs to promote his latest project, in this case a film called Miranda. It is the story of a librarian, played by Simm, who falls in love with a mysterious stranger, played by Christina Ricci. Actors, says Simm, need publicity. Well, of course they do. An interview is a transaction. The journalist gives publicity and the celebrity gives a little tiny piece of themselves. So for either side to want one without the other is just not on.

"I’m not into chit-chat," Simm says. "I can’t be arsed, you know?" Yeah, I did know, actually. It wasn’t the first interview of the day. It was the end of the afternoon and it all seemed so pointless that I was considering packing up early and heading for the airport. And that would have been a mistake.

FIRST IMPRESSIONS. He comes in the room, a slight, almost furtive figure in a baseball cap. Right away there’s the diffidence. Perhaps it’s shyness, perhaps simply animosity about being in a situation he doesn’t really want to be in. He has a moodiness, a nervous intensity, that becomes charisma on screen. By the end of the interview, he has made me interested enough to raid the video shop. He’s been in Human Traffic and Wonderland. But he’s also a well-known face on British television and recently took his place alongside James Nesbitt and Jonathan Ross in a "Hot 100", as voted for by television experts. He was Danny in The Lakes, Raskolnikov in Crime and Punishment, Cal in the recent political thriller State of Play (in which he played, ironically, a journalist), and then, in one of last month’s BBC adaptations of Chaucer’s Canterbury Tales, Simm became Ace, a prison inmate who falls in love with his literacy tutor.

Ace was perfect for Simm. Smart, streetwise and complicated. Big baggage and big emotions. It was obvious that any actor who could pull off that combination of toughness and vulnerability had to be interesting. He didn’t just play the emotions; he understood them. "You can tell when people have simply learned lines," he says. "It’s just like a parrot. You can see it in people’s eyes if they are feeling it. You can’t really lie then."

He grew up in the small northern town of Nelson, near Burnley. "It was literally cobbled streets and scuffed knees. It was just a really small town, one of those places you want to get out of. I am very proud of being from the north but I don’t think I could have achieved anything staying in Nelson. If you’ve got any kind of ambition you need to go quite quickly or you’ll get stuck there."

His father performed in working-men’s clubs and Simm used to sing with him and play guitar. "If you can entertain people in those clubs, you can entertain anywhere," he says. But he admits he wasn’t a natural performer. "I would stare at the floor and never smile and my dad would constantly tell me to smile on stage."

It must have had an effect on him as a child, going round men’s clubs when his pals were out playing. He doesn’t know, he says, just another in a long line of "don’t know" answers. It wasn’t music he saw as his passport out of Nelson. It was acting - acting was easier because you didn’t have to be yourself. "I’m quite shy being myself. As an actor, you can pretend to be other people. You’re hiding behind another character. I’m not shy when I’m acting, not at all. Otherwise I wouldn’t be able to do it. I’m just shy when I walk down the street and people go, ‘It’s you, innit? Innit?’ I just think, ‘I am glad you like what I do but I don’t know you and I don’t particularly want you to know me.’" He prefers it when they say they loved his performance and keep walking.

He enjoyed the clubs in a way but as he got older he wanted to be in a rock band. He got opportunities but, he says, "I couldn’t leave my dad." Later, he joined Magic Alex, a band that reached the dizzy heights of supporting Echo and the Bunnymen. He still performs with them periodically. He sighs. He’s talked about music and the clubs such a lot in interviews. His dad wants to know why he keeps talking about it and he has to tell him it’s because reporters keep asking about it. All right. Tell me about your mum instead. Did she work? "Yeah, yeah. I can’t remember what she did then. Nothing specific." What was she like? "She’s great, my mum." Well, that narrows it down: Mrs Simm of Nelson, great in a non-specific kind of way; worker in a non-specific kind of employment.

Let’s try acting techniques. He had told me earlier he was a good actor. What makes him good? "I don’t know. I really don’t." He must. Well, he says, he has a box of tricks. So what’s in his box? He’s not going to tell me that. Is he afraid to define his own talent? "I don’t want to shatter the illusion." Right. Fatherhood? His two-year-old son Ryan has changed his life. "Probably saved my life." That’s interesting. Did his life need saving? No, no, forget he said that.

Later, I worked out where the switch flicked - it definitely happened when we gave up on the interview and tried conversation instead. But, really, the big change came when he talked about Ryan. "Got kids?" Simm asks. Yeah. "Great, innit," he says with a new level of animation. It becomes apparent that not only does Simm adore his son but Ryan has changed who he is. "You don’t want to die. You cross the road differently. Everything changes. I’m less selfish. You are not number one any more. Something is more important than you."

He is 33 now but his 20s were one long party. He was successful as an actor, successful as a musician. "I had a mad life. I was out all the time getting trashed. It was a great ten years but something needed to root me to the ground and give me responsibility." He took drugs in those years and friends later gave him stick for his involvement in the government anti-drugs campaign. But the point for Simm was that the message was drug education, not a simple "say no". "I’d never tell Ryan not to take drugs. I’d leave it up to him. I’d want him to be able to tell me about what he was doing."

At 18, he took cannabis because he wanted to hear music more keenly. "I had my first spliff lying on the floor listening to Sergeant Pepper. It made music sound better." When he took the part of Jip in Human Traffic, he also experimented with Ecstasy. "If I hadn’t taken it, I’d have got it wrong. You have to know what you’re talking about." He’s not encouraging anyone to take drugs, he says. "But, statistically, you are more likely to die choking on a cabbage leaf than taking E." Later, watching Human Traffic, I discover this is one of his lines from the film.

But things have changed now. He has lost his self-destructive streak. "That’s what I meant when I said earlier that Ryan saved my life," he explains. "I’ve got it in me to be a real hedonist, to be the last man standing." During those years, he had a relationship with Spice Girl Emma Bunton which is always described in the papers as an "unlikely" affair. Unlikely, presumably, because Simm hates the whole celebrity circus. He and Emma couldn’t have been another Posh and Becks, then? To be fair, he handles the mischief of the question well. He grins. "No," he says.

His partner now is actress Kate McGowan and they juggle their careers round Ryan. Since State of Play and Canterbury Tales, McGowan has worked and he has looked after Ryan. "It’s hard work - really hard work," he says ruefully. It’s a double-edged sword, two actors together, but at least they understand the job. Sex scenes, for example: if McGowan has to film a love scene, he understands and doesn’t ask about it, just as she does with him. But he once had a girlfriend who started gripping his hand when watching him on screen with another woman. "I thought, ‘It’s not real,’ you know? It’s horrible. You can’t have that kind of relationship."

Sometimes when he looks in the mirror now he sees his father. But parenthood has given him an insight into his own parents. He has been away from home 15 years but only now understands what his mother and father felt when he left. "Sometimes, I’d ring my mum and dad and there would be massive pauses and I’d say, ‘You’re not speaking any more,’ and they’d say, ‘I just want to hear your voice.’ Now I know. Or when I went back to college after a weekend they would cry and I would think, ‘Oh, for God’s sake. I’ve been gone for ages; what’s the matter with you?’ Now I know. It must have been heartbreaking."

He first went to college in Blackpool. It says more about Nelson than Blackpool that he thought the seaside town was really posh. But then he moved to London and took a place at the Drama Centre. RADA, he thought, was for luvvies. "It was at the point where I was really into Marlon Brando and Robert De Niro and I really, really wanted to go through the mill. When I finished I had a sense of achievement. Everyone who goes there has that. When I spoke earlier about the box of tricks I meant technical things and thinking things, emotions you can get at different times for the camera. Drama school helped me do that."

Not that he always realised it at the time. In his second year he’d had enough. "I wanted to do it so quickly. I got so frustrated that I wanted to leave college and just do it. I went and auditioned for Blood Brothers and went up to the office and said, ‘I’m leaving. It’s bollocks. I can’t be arsed. Bye.’ I walked to Camden that day. Naked was on there, Mike Leigh’s film, and I went in to watch that. I was totally inspired. It was incredible. The intensity of the performance. Two hours later I went straight back and said, ‘I’m back.’" Maybe it was arrogance to have gone off in the first place but there’s a real level of humility to acknowledge the mistake within hours rather than try to save face. "It was frustration but I think you’re right about the arrogance as well. I thought, ‘I can do it.’ Then I watched that and thought, ‘Oh, maybe I should go back to college!’" But Simm does believe in himself. "You just had to have confidence in yourself and I had that. I knew I was a good actor and it was about proving it. I always had an unshakeable confidence in my own ability. If that gets shaken, you’re not going to make it. There are so many people going for the same part."

Not that his family always appreciates that. He comes from a family of five, with two younger twin sisters. Perhaps Simm was always the outsider. Five years older, he left the house before he really got to know them. Certainly, acting is a different world. "I say I am doing a part and my mum will say, ‘Oh, why didn’t you do the other part?’ I think she thinks everybody gets a play. I’ll say, ‘Mum, you do know that thousands of people auditioned for this part, don’t you?’ And she says, ‘That’s great, love.’" He smiles.

Despite Nelson, his own interest kicked in after seeing Rebel Without a Cause. He went straight to talk to the school drama teacher and started classes. Acting still has that power to pull him emotionally. That time in the darkened cinema watching Naked, for example. "At the end, at that moment when the screen goes black, I just sat there and went, ‘Wow!’ When I went to see Taxi Driver and The Last Temptation of Christ I got the same thing."

His emotional responses to film recently have been partly shaped by Ryan. Have you seen Etre et Avoir? he asks. I haven’t. It’s a documentary about a teacher in France. Later, when I look it up on the web, there are glowing tributes from critics saying if you don’t have a tear in your eye at the end of it, there’s something wrong with you. Nothing wrong with Simm, then: "It made me cry. It’s just amazing watching the way he deals with the kids - his patience and the way he talked to them and his discipline. It’s just wonderful to see. I wouldn’t usually cry at something like that but now anything to do with kids…"

Like child pornography, he says. "I just can’t…" He is lost for words. "I just can’t deal with it. It makes me angry; really, really, really angry. I can’t tolerate that." We agree there is almost always something to empathise with in people who have committed a serious crime, even murder. But paedophilia is different. "I think it would be difficult for me to play such a part," he says. "I always have to find something in the character to empathise with, something I can make likeable in some way. I don’t think I could do it." It seems more a strength than a weakness, this admission that art cannot always take precedence over life.

Simm’s moodiness is layered - interesting, multi-faceted layers that throw up richly coloured emotions when the light hits them. If I’d gone early to the airport I wouldn’t have seen any of the patterns or colours. But the moment I really see what John Simm is made of is when I suddenly realise with panic that the quiet club we came in to is now very noisy. I’ve been so absorbed in the conversation I forgot about keeping a check on the tape levels. The slim machine sits reproachfully on the table. The man who hates interviews looks at me. "Don’t worry," he says, kindly. "We can do it again if it hasn’t come out. Really. We can do it again."

JOHN SIMM says at the start that he doesn’t want to give himself away. "People can have me in their living-room. But I don’t want to give them me." Which is a shame because you give part of yourself away in an interview whether you want to or not and resistance just means you give the worst rather than the best. Anyway, as an actor he gives himself away all the time, doesn’t he? That’s part of creativity. Artists reveal themselves in their paintings; writers in their novels; actors in their films. Doesn’t he reveal himself on screen? "Yeah," he says thoughtfully at the end of the interview. "Yeah, absolutely. When I cry, those are real tears - that’s how I cry. When I kiss, that’s how I kiss. That’s opening yourself up. I’ve never really thought about that."

In the end, there is something almost uplifting about interviewing John Simm, a nice reminder that cynicism doesn’t always win, that you can think the worst but the best will force its way through anyway, like grass through cracks in the concrete. The transaction becomes an enjoyable conversation. Out of irritation and hostility and zero expectation comes a surprising empathy. That film Etre et Avoir, I should see it. "You’d love it," he says with conviction. "You would love it." I don’t doubt his judgment for a minute. Two hours after we meet, the only reason I don’t stay for another drink is that if I do, I’ll miss the plane I was going to be early for. Funny old world, innit?

• Miranda is released on November 7