She’s had an agent from the age of six, so perhaps it should be no surprise to learn Keira Knightley is maturing into a confident actress

She has most recently stared down mental illness in the Freudian drama A Dangerous Method alongside Viggo Mortensen and Michael Fassbender, infidelity in Last Night, war in Atonement, rape in London Boulevard and death by organ harvesting in Never Let Me Go.

Is it any wonder that when she was offered the part of a free spirit in the comedy Seeking a Friend for the End of the World, the recently engaged 27-year-old couldn’t say yes fast enough. “I have been doing a lot of dramas, real tragedies, and I love doing them,” Knightley explains.

“But I definitely wanted something that was a little bit more upbeat. As my mum (Scottish playwright Sharman Macdonald) said, ‘You choose upbeat and it’s a movie about the end of the world.’”

Knightley laughs. “I mean, I know. I’m working my way towards the really positive. But I have to say, it was just lovely doing something so different.”

In Seeking a Friend, an anxiety-prone insurance executive named Dodge (Steve Carell) strikes up an unlikely bond with his neighbour, the impulsive Penny (Knightley). Their relationship takes on a sense of urgency thanks to the fact that they’re hooking up a few weeks prior to the arrival on earth of a 70-mile-wide asteroid.

“I read the script and I thought, ‘I’ve never read anything like it,’” says Knightley. “It’s a unique take on the end of the world. I found it incredibly positive and uplifting.

“I thought the vibe of the piece is what becomes important, and it’s about love, friendship, companionship and the wonderful, small moments [of life].” While the actress is usually grappling with tough issues in her movies and Carell is cracking up in his, this time around, she’s doing her best to keep his spirits buoyed.

“I knew [Keira would] be amazing and stunning and super-smart,” says the film’s writer and director, Lorene Scafaria. “But here’s the surprise: she is so damn funny. So there’s this refreshing blend of Keira, known as a dramatic actress, being more of a comedienne, and Steve, known as a comedic actor, doing a more dramatic role.”

Knightley enjoyed playing “an amazingly positive woman,” she says.

“Penny is somebody who’s completely able to say, ‘This moment is wonderful.’ And there are so few people like that. I’m not like that. You know, I’m very easy going, ‘Oh God, I’m really unhappy.’ But she’s like, ‘This milkshake is brilliant, the sun is brilliant.’ I just loved that about her.”

Knightley’s personal life has turned a very positive corner recently, what with her engagement to Klaxons vocalist and keyboard player James Righton, whom she has been dating since February last year. And she was bridesmaid at her brother Caleb’s wedding last April, when he married Scottish fashion designer Kerry Nixon at Pollokshields Burgh Hall in Glasgow.

But while Knightley’s character in the new film chooses to live life to the fullest, even as she’s nearing possible death, the actress imagines she’d have a much different reaction to news that the world was ending.

“I think I’d be crying and terrified, sitting in a corner,” she says. “I’d love to say that I’d be at a party and having a really great time. But, honestly, if an asteroid was about [to crash into] Earth, I think I’d just be going, ‘Is this going to hurt? I want to hide’.”

As for bucket lists and last requests, Knightley says she’d only have one activity on her mind. “The only thing that I’d want to do is to be with people that I love,” she notes. “I don’t think I’d want to do anything else.”

One of the reasons Knightley was so eager to sign up for Seeking a Friend was the opportunity to work alongside Carell. “Steve is amazing,” she says. “He’s such a lovely man. One of my favourite performances of his is in Little Miss Sunshine. That’s a genius performance because he’s completely believable as this horrendously depressed, suicidal guy, and yet he’s still incredibly funny within that. It takes talent to be able to do that.”

According to Knightley, shooting the film was a breeze, with the exception of one  sequence that required her to drive and act at the same time. It’s a trick she had never tried before – and is likely to never try again.

“I nearly killed Steve,” she says. “See, I don’t drive that much. I’m actually a good driver but I don’t have a car in London.

“We were driving away from a riot [in the film]. So, I was acting that. I was meant to be a bad driver. Unfortunately, I was a very bad driver. We had Steve in the back of the car and the cameraman in the front of the car. And I just slammed on the brakes and it sent Steve flying into the cameraman’s crotch. It’s a miracle if the cameraman can still have children. I was so sorry.”

The daughter of actor Will Knightley and actress-turned-playwright Macdonald, Knightley has had an agent since she was six years old. But it wasn’t until 2003, when she appeared opposite Orlando Bloom and Johnny Depp in the Pirates of the Caribbean movies, that she became a household name. Since then, she’s worked steadily in movies as varied as historical epic The Duchess, the action-packed Domino and the fantasy epic King Arthur. Her next project is among the most ambitious of her career.

Teaming for a third time with her Pride and Prejudice and Atonement director Joe Wright – the first of which won her an Oscar nomination, the second a Golden Globe nod – the actress takes on Leo Tolstoy’s classic of despair, Anna Karenina, scripted by Tom Stoppard and starring Jude Law as Alexei Karenin and Nowhere Boy’s Aaron Johnson as Count Vronsky.

The film, due for release in September, was very much in Knightley’s comfort zone. “Tragedies are probably my favourite things to do,” she admits. “But they do take it out of you. It was very difficult to shake Anna off at the end of the day.”

Knightley, who filmed her first nude scene when she was only 15 in The Hole, says unlike many performers her age, she isn’t compelled to work for the sake of keeping busy.

“If the role isn’t good enough, I’m not going to take it,” she says firmly. “As I’m getting older, I’ve been offered some roles and – even though I’m not thinking about having kids right now – I suddenly thought, ‘If a daughter of mine at any point watched me doing that, would I feel really ashamed of myself?’

“I mean that more in a feminist way of, ‘Is that part good enough? Or am I going to be playing a character that just turns up and takes her clothes off?’

“I’m not willing to do that even if it’s a cool movie and it’s a cool director. I want it to be more than that.”

• Seeking A Friend For The End Of The World is  released on Friday