Frankenstein: Man or monster?

Danny Boyle is not a man to shy away from a challenge. The proof is all over cinema screens in his latest film 127 Hours, and it's evident in his return to British theatre, where he cut his teeth as a director in the 1980s. Boyle's choice of play – an adaptation (by Nick Dear) of Mary Shelley's Frankenstein, opening at London's National theatre next month – might be conventional, but not the way he is presenting it. Two actors will alternate the lead roles – a ploy attempted rarely, possibly with good reason.

"Frankenstein is creating life without women," Boyle reasons. "The idea is to bring two actors as close to that notion as possible. And how do you do that? In terms of the performance, Frankenstein and the Creature literally create each other: every other night they reinhabit each other."

Boyle's one problem was: "Could we find two actors who could do it?" Through auditions he chose Benedict Cumberbatch, who last year became one of Britain's leading young actors, both in the theatre – he was impeccable as a lethally bored socialite in After the Dance – and on TV, as a fiercely modern Sherlock Holmes. A rifle through his own back catalogue led Boyle back to Jonny Lee Miller, who played Sick Boy in his 1996 film Trainspotting. It would be easy to assume that director and actor were marking time until the opportunity arose to collaborate again: in fact, says Miller, "I hadn't had any contact with Danny since then."

Because this experiment is conducted so rarely, the three weren't initially sure how to work together. "One consideration was that they might rehearse separately, unless they were in the same scene," says Boyle. But as Cumberbatch and Miller befriended each other, they found this unhelpful. "We're not precious," says Miller. "We find it constructive to talk to each other about what looks good, what doesn't. We're more of a team."

"The dialogue between us is selfless and co-operative," adds Cumberbatch. "If there's something really good that he does, I will ask if I can incorporate it." Miller is anxious to point out that they're not stealing ideas from each other, but Cumberbatch has no such qualms: "There's no shame in stealing – any actor who says he doesn't is lying. You steal from everything."

Chatting in a room at the National that gazes across the river to St Paul's, the pair are so enthusiastic about their partnership, you wonder if there are drawbacks to alternating. Their answer is unanimous and instantaneous: "Time" – that is, the lack of it. "It's double the workload but not double the rehearsal time," says Cumberbatch. And it is much harder to learn lines. Cumberbatch says that he will start learning Frankenstein's lines, "then get wound up into the Creature's, because he gives cues. Having both characters on the same page is a distraction."

Neither actor claims to find one or other character easier or preferable. And it's Boyle's intention that Frankenstein and the Creature should "become the same in a way: two strands of the same part". And yet, the thought occurs that the actors' strengths veer in opposite directions: that Cumberbatch is obvious for Frankenstein, and Miller will excel as the Creature. That's more of a compliment than it might appear: it's Boyle's intention to make us rethink a character "distorted" by cinema, and "give him back his voice". As Miller says: "People think this is a horror story about a monster, but it's not: it's about abandonment, companionship, prejudice, and original sin – are you a product of your environment, is evil within you?"

This idea of a division of talents partly arises from the evident differences in their personalities. Miller is softly spoken and modest to a fault; fearful that anything he says about acting will make him sound, in his words, "fucking pretentious". Cumberbatch, by contrast, is dizzyingly confident, second-guessing questions and impersonating his director, quick to skewer himself as a "pompous" actor. What they have been surprised by in each other is telling. "This is awful, because it reveals more about me than Jonny," says Cumberbatch, "but he's very old-fashioned: he's very polite, a good timekeeper, concerned and caring." And Cumberbatch, says Miller, is "incredibly clever, incredibly inquisitive. He won't settle: he needs to know exactly what's going on. He's always asking, 'No, but ...' It wakes you up; it makes me do the same."

The two men are fairly close in age – Miller, at 38, is four years older – yet their careers have been utterly divergent. Since shooting to fame in Trainspotting in 1996, Miller's career has been chequered, with ups – a lead role with Sienna Miller on Broadway last year; the 2009 film Endgame – and downs, with films never seeing the light of day and US TV series being unexpectedly pulled.

Miller exudes quiet contentment with his lot: he is married to actor Michele Hicks, and they have a two-year-old child. But he admits that, as he progresses through his 30s, work is harder to come by, and for six months last year there was nothing at all: "Nothing of interest. And then you start panicking – you go a bit fucking crazy." Looking back on Trainspotting and everything that happened after, he thinks he had fun – after all, he was briefly married to Angelina Jolie – but, as far as work went, "I didn't really capitalise on it in the correct way."

Cumberbatch's progress was slower and steadier, until 2010, when he shot into the limelight with Sherlock and a call from Steven Spielberg, who wanted Cumberbatch to appear in his adaptation of War Horse. It would be easy to think Cumberbatch has had the better ride, but he's not sure: apart from anything else, "I should be so lucky to have married Angelina Jolie. If I'd had fame early on, I'd have been able to abuse it in the way that a young man should. I've been working to this, but a lot of the fruits of it I can't really enjoy."

He's not being wholly facetious: essentially, Cumberbatch is too mature to take advantage of the "ridiculous, very flattering adoration" that he has attracted since Sherlock was screened last August, but nor does he feel that he is in a position to settle down. "I used to be very, very broody – I still am, but it kind of upsets me. I'm getting more busy than I ever have been with work, there will be travel involved, and this is probably the worst time to think about having kids – just when I'm financially secure."

He's wary, too, of the implications of fame. "You get known as 'Sherlock'. That's not just from people who can't be bothered to remember 'Benedict Cumberbatch', and who can blame them, but because it's such a strong signature." Although he has signed up for a new series of Sherlock – he leaves to start shooting it the day he finishes in Frankenstein – the last thing he wants as an actor is to be "a reason to sell Sherlock T-shirts". So he consoles himself with one thought: "No one calls George Clooney 'Doug Ross' any more."

Far better for Cumberbatch to be remembered as a great Frankenstein – or should that be a great Creature? Time will tell whether he and Miller successfully pull off both roles, or dazzle in only one. Which raises the question: are audiences effectively compelled to see this production twice? "Not at all," says Boyle, laughing. "It's not a marketing tool."

As part of National Theatre Live, on 17 March Frankenstein will be broadcast live to cinemas around on the world.