Black Watch: 'One of the two great Scottish plays of the past 500 years' returns, but the all-new cast are determined not to coast on that reputation

The dialogue transports me back to a Drill Hall in Edinburgh at the beginning of the Fringe in 2006. Back then, Black Watch was a daring experiment which looked like it might be a hit. The National Theatre of Scotland (NTS), which created it, was just six months old. No-one could have predicted the future.

Four years on, Black Watch has been seen by 113,000 people in three continents and won 22 awards, including four Olivier Awards. It has been showered with superlatives. Historian Michael Fry reccently claimed it was one of two great plays Scotland has produced in the past 500 years (the other being Ane Satyre Of The Thrie Estaites). It has won the support of stars from Cate Blanchett to Sean Connery.

This autumn Black Watch returns with, for the first time, an entirely new cast. The original creative team are back: writer Gregory Burke, director John Tiffany, movement director Steven Hoggett and music director Davey Anderson. Tiffany says this is "a testament to how important it is to all of us and how passionate we are about it. Normally, by this stage, you'd get the deputy of the deputy."

The Glasgow rehearsal room is a place of shaved heads, army fatigues and replica assault rifles. Tiffany fine-tunes a scene in which the squaddies are stranded in the desert when their armoured transport breaks down: "Who's worried about how exposed you are?" he asks the young cast, many of whom are just out of drama school. "How do you communicate that?"

Black Watch is based on interviews done by Burke with demobbed soldiers in Fife, and is based on real events which took place when the regiment was deployed to Camp Dogwood in 2003. It stood out among all the plays about the Iraq war because of the way it combines strong acting and realistic dialogue with the in-your-face theatricality of the singing and Hoggett's demanding choreography. The cast train with him daily; each show requires a two-hour warm-up.

In rehearsals, Tiffany and Hoggett patrol the edges of the performance space, watching the action shifting between Iraq and a pub in Fife where the squaddies are being interviewed about their experiences. At one point the pool table gets stuck and two of the technical team (two of the only women in the room) have to climb underneath to fix it. Tiffany calls a halt to a scene when he forgets a flak jacket. "Come on!" he grins. "It's not like I've done this before!"

For the sake of box-office takings alone, it makes sense to revive Black Watch. This production will tour for nine months, taking in the Ulster Bank Belfast Festival, a second London run and new destinations in the United States, including Washington and Chicago.

But already it feels like history: the army has left Iraq; the Black Watch regiment has been all but lost in army restructuring; even George W Bush is a distant memory. In an interview in 2008, Tiffany said that the play could not run indefinitely: "I think at a certain point you have to say enough is enough."

Not yet, though. When we meet he says it is as relevant as ever. "That's the sad thing. Afghanistan is even harder than Iraq for the soldiers, but very similar in terms of what modern warfare is. And Iraq? That's all solved, isn't it? We did a great job! When I saw Tony Blair being interviewed by Andrew Marr the other day, I just thought: 'I was absolutely right to do this.'"

He knows that expectations for the play will be high. Black Watch's reputation precedes it, and then there are those who have seen and loved it and want to see it again. Apart from minor pieces of "streamlining", his aim is to change as little as possible.

Hoggett, however, has made the choreography even harder. Tiffany says he left the actors under no illusions about what they were letting themselves in for. "I know what it takes for an actor to be in Black Watch, I know what demands it makes of you. I tell everyone this before they sign the contract – it's what I call my pre-nup. 'You're not just an actor, you're an athlete and a singer. It's two hours long, and you get through a litre of water at least in that time. I'll make sure you get the training and the support you need to do that, if you promise me that you won't go out and pish it up the wall after the show.' I become quite tough."

Jack Lowden, who leads the cast of squaddies in the role of Cammy (the part played by Brian Ferguson in the original production), is one of those who has had to adjust to Hoggett's training regime. He is just 20 and Black Watch is his first professional job; he will miss most of his final year at RSAMD to do the tour, but it will count towards his course credits.

"I'm not going to lie," he says. "In the first couple of days it was tougher than I thought it would be. I think we all thought that – it was a shock. Now, after three weeks, it's still difficult, but our pain threshold is a lot higher, we are becoming fitter. Once you've done a couple of run-throughs, you understand how much you do need your physical fitness." He saw Black Watch for the first time in 2007 on a school trip with his Standard Grade English class. "It just blew me away, especially the realism of the dialogue between the guys, I was immediately drawn in by that. I'd already planned to go into acting, but seeing Black Watch cemented that."

Tiffany says when Lowden first auditioned he had no idea how young he was and recalled him several times to check he was up to the demands of the role. Lowden recalls the moment he was cast with disbelief still in his voice. "It was surreal. It's something you dream of doing, like playing for Scotland, it's on that level."

With young actors beating down the door to be in Black Watch, the biggest challenge Tiffany faced was finding one who could play the bagpipes to the necessary level. "We tried everybody. Then Anne Henderson, the casting director, suggested that instead of looking for an actor who can play the pipes, why don't we try looking for a piper who can act? She saw between 30 and 40 pipers and then got them to do an acting audition – I called it Pipe Idol. That's how we found Cameron Barnes – he's 20 and comes from Methil."

"I don't know if there's a young Scottish actor who wouldn't want to be doing this show," says Scott Fletcher, who plays Kenzie. Fletcher, 22, is following in the footsteps of his brother, Ryan, was in the original Black Watch cast. Ryan, with fellow Black Watch-er Henry Pettigrew, is currently touring in the NTS's Beautiful Burnout, directed by Steven Hoggett and Scott Graham.

"Black Watch has been my favourite show since I saw it at the Fringe in 2006," Fletcher says. "It's one of these shows that makes you want to be an actor. I think it confirmed to me that's what I wanted to do with my life. John, Steven and Davey have been great with us, taking us through the show to give us that sense of ownership over it, so it's our show, we don't feel as if we're replacing anyone."

He swapped one set of fatigues for another, coming straight to Black Watch from filming BBC2 Scotland's sitcom about squaddies, Gary Tank Commander. They filmed the programme's "flashbacks" (made to look like mobile phone footage posted by soldiers on YouTube) just up the road in Glasgow's Maryhill – "it doubled as Helmand Province, which is funny" – just before Black Watch rehearsals began.

Keith Fleming, who won a CATS award for his performance in the NTS/Dundee Rep production of Peer Gynt, says he "grabbed" the opportunity to be in Black Watch in the role of the Sergeant/Writer, "although your idea of how fit you are and the reality is something different, you realise that in the circuit traing." He says a revival of a play needs to be as strong, if not stronger, than the original. "If a play's worth doing, it's worth doing to the best of its capabilities. You don't just say, 'We had it right the last time, let's do it again.' Black Watch has proved its worth and will continue to do so. It can't just live on its past glories. We have a duty to reach those levels – if not better them."

• Black Watch is at the SECC, Glasgow, until 9 October and the AECC, Aberdeen, 13-23 October. For other dates see www.nationaltheatrescotland.com