‘Oompa Loompa juice,’ says the actor Alex Jennings when I ask if he takes any supplements to preserve his looks. He’s 57 but could pass for a decade younger. We meet backstage in his Drury Lane suite, which boasts a fridge crammed with pink champagne, where he’s preparing to play the lead role in Charlie and the Chocolate Factory. His relaxed demeanour and silky voice create an air of instant geniality that is reinforced by his towering figure. He’s six foot four and as lean as a fast bowler. Though he’s due on stage in 90 minutes, he lounges semi-horizontal in an armchair showing no trace of anxiety. ‘I do get nervous before the first big number. The great thing is, once the music starts, it don’t stop.’ Initially, when he took over from Douglas Hodge in May, he admits to being beset with nerves. ‘There were moments of thinking, why the fuck am I putting myself through this? Why am I doing this? I could just walk to the stage door, get in a fast car and say goodbye to it all.’

He’s still experimenting with the Wonka character whom he describes as ‘a child killer. He’s weird. Children disappear [in the show]. But that’s the misanthropic world of Roald Dahl. It’s very dark, but that makes it more interesting to play. I don’t feel settled yet. I’m still looking for new funny voices to do. He has a line about “all the voices in my head”, so that’s my justification for doing my favourite voices. I do Cary Grant, Noël Coward and Katharine Hepburn. Fleeting, fleeting. And all from a core of truth.’

He’s ‘in awe of’ the kiddie thesps playing alongside him. ‘They’re lovely. There are no ghastly little stage monsters. They’re delightful and so talented and totally unfazed. That is gobsmacking.’ He says the reaction from the audiences is enough to vindicate his entire career. ‘They go bonkers. It’s really gratifying. That’s what we went into it for: for people to stand and cheer and tell you that you are a valid human being.’

He’s contracted until next summer and he knows all about the long haul. In 2002 he took over from Jonathan Pryce as Henry Higgins in Trevor Nunn’s production of My Fair Lady. Playing the same role for 11 months became ‘a pretty exhausting disciplinary challenge’. Producers insist that stars take holidays to help recharge their batteries but with My Fair Lady it made no difference. ‘I loathed it when I did Higgins. I hated taking the holidays. I didn’t want anybody else to do it.’

Looking back on his childhood he has no recollection of a light bulb going ping! and ordering him to take to the stage. ‘I was always a bit of a clown, my parents would tell you. I’d always mucked around with funny voices.’ An economics master who directed plays at school encouraged him to act. ‘He sort of changed my life, really. The classes he ran opened up a whole Pandora’s box of stuff.’

After university he trained at Bristol Old Vic and his subsequent career followed ‘a sort of gradual upward trajectory’ rather than giving him instant celebrity. Was he frustrated, in his twenties, that he wasn’t getting starrier roles?

‘I was. There were floppy-haired posh-boy contemporaries I was gnashing my teeth at. Yet another Merchant Ivory movie that I wasn’t in.’

He rejects the fashionable orthodoxy that poorer kids are finding it harder to get started in acting these days. ‘Not the case here. The young ones here, they’re from all over, that’s the impression I get. The hard thing is there are too many people wanting to do it.’ Given that he’s on the left of the political spectrum this view is surprising, but he describes his loyalties as instinctive or tribal. ‘I don’t kind of waver, really, but don’t ask me too many questions about why I’ve always voted Labour but I always have.’ His verdict on Ed Miliband isn’t likely to be re-tweeted by party headquarters. ‘We got the wrong one. I think they made a mistake.’

He’d like to perform more classics, and more Shakespeare, ‘because they feed you’, but he’s uncertain which roles to aim for. ‘The things I wanted to do, I’ve missed out on. Macbeth and Richard III have gone now, I think.’

Lear?

‘Don’t know really. I can’t see myself in it — yet. I was offered Prospero a couple of years ago and I didn’t know what I would do with it. It didn’t speak to me, whereas when I was asked to do Hamlet, and Leontes in The Winter’s Tale, I jumped at them. I’ve never quite got The Tempest.’

Screen opportunities beckon. In the autumn he’ll abandon Charlie for two months to film Alan Bennett’s The Lady in the Van with Maggie Smith. ‘I’m playing Alan.’ Although he’s the star, he says it was the ‘combined powers’ of the director Nicholas Hytner and Dame Maggie that secured the green light for the film. Is he hopeful it might turn into an offbeat Brit-com hit in America?

‘Yes,’ he says simply. ‘Do you have a large transatlantic presence to build on over there?’ My phrase comes straight back across the net. ‘I don’t think it’s very large, my transatlantic presence. I did Hamlet with the RSC 17 years ago, which we took to Washington. And I did an RSC tour of A Midsummer Night’s Dream in ’96. I would love to work there more. I could imagine having another go at sitting in LA for a bit. I did that once. A couple of months, and I felt myself going backwards.’ The roles he was offered were ‘ridiculously inappropriate’.  ‘So I came scuttling back.’

Did he not get a stab at a cockney gangster?

‘No. I’d love to. Mostly I play posh blokes.’

With Sam Mendes directing the forthcoming Bond movie, would he be interested in playing Bond’s evil adversary?

‘I’d love to play a Bond villain and be in one of Sam’s projects, because I’ve known him for a very long time and we’ve never managed to do anything together.’

But hang on. Sam Mendes is the director of Charlie. He must have taken charge of rehearsals as Jennings learned the role and familiarised himself with the company. But it transpires that the task of initiating the novice was entrusted to a pair of deputies. I sense a bone of contention here.

‘Oh, no,’ comes the pebble-smooth reply. ‘They were absolutely marvellous.’