Once more with feeling

"I love it," he exhales. "This is my fourth show here and it just keeps getting better. It's such a brilliant city."

The Black Watch director is basking in the rapturous reception for his musical adaptation of the much-loved film, Once, which charmed audiences worldwide in 2007 and was a phenomenal hit in the US. Shot on a shoestring in Dublin, the independent picture won an Oscar for its song Falling Slowly, the enchanting courtship between a busker and Czech immigrant heightened by the fact that its leads, Glen Hansard and Markéta Irglová, fell in love during filming.

Guided by the original screenplay rather than the hastily-shot, heavily- improvised film itself, playwright Enda Walsh has also shifted the narrative to a post-credit-crunch Ireland, with the bulk of the story taking place in a traditional Dublin pub. As in the film, the score tells the story and the cast of actor-musicians are in the fullest flow of a hootenanny as the audience enter, playing Irish and Czech folk songs "arranged by our brilliant arranger Martin Lowe to be more Mumford and Sons and Belle Brigade," Tiffany enthuses. Punters can even buy a pint at the bar.

The reviews had only just begun appearing, the Hollywood Reporter claiming "this bewitching stage adaptation arguably improves on the movie, expanding its emotional breadth and elevating it stylistically while remaining true to the original's raw fragility", when the show's producers confirmed that Once would be transferring from the East Village's scruffy 200-seater New York Theater Workshop to the 1000-seater Bernard B Jacobs Theatre on Broadway in February.

Tiffany, who as National Theatre of Scotland's associate director returns to Glasgow in June to direct Alan Cumming in a one-man performance of Macbeth, the NTS's first production of Shakespeare, is at pains to convey that the Bernard B Jacobs is still a "surprisingly intimate and cosy" venue, a suitable cocoon for this ethereal romance.

"It's such a beautiful, humble, fragile film," he marvels. "The film director Pavel Pavlovsky said to Enda that it'll be like 'catching a butterfly'. And I really kept that in mind while working on it – it's incredibly pure and simple as a piece of theatre and the audience are responding to that. I'm delighted that it's coming to Broadway because it's a show that I would have been happy for it to simply live in the Traverse Two in Edinburgh."

Although he craved Hansard and Irglová's approval, "because their music's incredible", he knew he couldn't hope to recreate the naturalism of non-actors improvising dialogue as they genuinely fell for each other.

So as Girl, New Jersey-born actress Cristin Miloti is altogether feistier and more direct than Irglová. Despite never having seen the film, Tiffany assured her that she was under no pressure to do so, having only watched it once himself after he was approached to direct. Even so, Steve Kazee (Guy) is one of several in the cast who employed a voice coach's services to refine his Irish accent.

Tiffany was "keen that the show's whole vocabulary was that of actor-musicians", with everyone in the cast capable of playing at least one instrument. "It's a story about music's healing power," he asserts.

Hansard and Irglová, who subsequently split yet remain friends and continue to perform together as The Swell Season, "came into rehearsals a couple of times and gave us little pointers here and there about where the songs had come from. But they're musicians and we're theatre professionals, so we just got on with our own projects."

Growing up in Huddersfield, musicals were Tiffany's theatrical initiation. But he found himself reaching back to a more spontaneous tradition of performing in back rooms and bars. Breaking down the barriers between performer and audience "is a very working-class thing, where music is doing your 'turn', that idea of communicating things through music that you could never do with the mere words. That was absolutely my childhood in Yorkshire."

After recently embarking upon a fellowship at Harvard University, where he studied paralanguage, "exploring the notion of how language defines us and how we try to control the way the world interprets us through our speech", Tiffany analysed the speeches of Franklin D Roosevelt, Ronald Reagan, Barack Obama and even Adolf Hitler, intrigued by the way that "George W Bush cultivated a redneck Idaho kind of language".

"It'll feed into everything I do," he reflects. "I can't say how exactly, but it's going to feed very, very directly into Macbeth."

Recalling those dry, dusty interpretations of Shakespeare he was subjected to at school, it was at the 2007 Edinburgh Fringe that Tiffany declared an indifference to directing The Bard, claiming "we just don't need any more Macbeths in the world, however brilliant mine might turn out to be."

Chuckling now in recollection, he says "listen, we're allowed to say things and not mean them. This is Macbeth in the way I want to do it." Reuniting him with Cumming, after the Aberfeldy-born actor starred in the NTS' 2007 production of The Bacchae, Tiffany describes this multi-media production of the Scottish play in which Cumming plays every single role, as "a very particular reading based on the fact that we've all got lots of different energies and voices within us."

As Once has shown, he isn't fazed about making radical departures from hallowed source material. And Tiffany is confident the Butcher of Inverness can prove as robust as his butterfly.

"People have told me that it's sacrilegious to do a one-man Macbeth. But it's been around for 500 years. It's going to last well beyond a month of performances from myself and Alan Cumming. Everyone can just calm down."

Macbeth opens at Tramway on June 14; Once moves to the Bernard B Jacobs Theatre in February after its off-Broadway run at the New York's Theater Workshop run ends on 15 January.