The problem, in a word, is comedy. Suchet is about to take the stage as Sid Field, comic genius of the Thirties and Forties, re-creating not just his life but the performances that had wartime audiences crying with laughter and influenced generations of comedians to come.
'I've done most things in my career now,' Suchet says, sitting in his broom- cupboard of a dressing room. 'I've been wonderfully privileged. But never have I felt the insecurity of that fine edge of comedy. It either works or it doesn't. There's no middle ground.'
This uncertainty - that what got a laugh last night may not get one tonight - is a way of life for comedians. But Suchet is not a comedian. Indeed, on the scale of serious acting he is very much at the dark end, a specialist in brooding, tormented souls in whom an explosive violence lurks not very far below the surface.
That was true of the stage role that first got him noticed, as the wronged husband in The Kreutzer Sonata, a one-man play at the Royal Court in 1977, based on a short story by Tolstoy. A spell-binding piece of concentrated acting, it led to a series of performances in similar vein - notably as Iago for the RSC, Timon for the Young Vic, and, back at the Royal Court, as the professor in David Mamet's sexual-politics hit Oleanna (1993). All enabled Suchet to display his gift for intensity without mannerism.
And then there is his television work, as Sigmund Freud (for which he won a Royal Television Society Award for Best Actor), and the role for which he is most famous - Hercule Poirot, a meticulous study in enigmatic ingenuity. Lighter than most things he has done, Suchet's Poirot still has the buttoned-up severity that he does so well.
None of this would seem to lead naturally to the role of Field, whose innocent, joyous and very physical comedy placed him somewhere between Buster Keaton and Norman Wisdom. But it is Field's very lack of inhibition which appealed to Suchet. 'I believe it's a good time to really stretch my own boundaries as an actor,' he says.
Suchet describes the Field show as 'the biggest risk of my career', the sort of claim actors are apt to make whenever they try something different. But it does represent a huge departure, into material which, even in an age that has seen the rehabilitation of Bob Monkhouse, Suchet knows may not stand up.
The idea came from Brian Eastman, producer of Poirot, after he saw a television programme on the history of British comedy two years ago. It stressed the importance of Field to almost everyone from Peter Sellers to Eric Morecambe. But Field's death in 1950, when he was 45, meant he was virtually unknown to anybody under the age of 60. In the struggle Field had to gain recognition, Eastman felt there was a great story to be told, and that Suchet was the man to tell it.
The result is that rather than go for the easier option of doing Poirot on stage, which he can take up whenever he wants, Suchet has spent the past year subjecting himself to the task of researching Sid Field, and learning skills he has hardly ever needed before - singing, dancing, and the comic timing and technique which won Field admirers throughout the theatrical world.
Field's humour was very much of its time - music-hall, essentially, comprising sketches of sublime silliness which have stayed in the hearts of those who saw them. The warmth of his personality and his ability to manipulate an audience were such that he reputedly had only to walk on stage and pull a face to have them falling about. But even Suchet admits that much of Field's material is dated and 'can read terribly on the page'. There is also a campness to his act which hasn't been seen since John Inman was last mincing about and which modern audiences may not enjoy seeing again.
'But what you have to remember is that Sid was never pointing fingers at people,' Suchet says. 'This is knockabout fun without any side. His characters were there to entertain, in his own unique style. There are two aims with the show, really. There's the intellectual mission, to show people who Sid Field was and why he was important; plus to bring to London the sort of show that hasn't been seen since the 1940s.'
Suchet says he has 'always given himself the right to fail', which perhaps explains why this has happened to him so little. He is the son of a London gynaecologist, went to boarding schools in Kent and Somerset (which he loved), and then to Lamda. Premature baldness, haunted features, brown eyes, and a beautifully modulated baritone meant he was cut out for sombre, disturbed characters.
His progress since he got his first part at the Chester Gateway in 1969 seems to have been remarkably smooth, and he has an air of purpose and generosity of spirit that suggests someone at ease with what he is doing. He is married to the actress Sheila Ferris. They have a son and daughter, aged 13 and 11, and live in Pinner. Suchet's brother John reads the news on ITV.
At 48, Suchet may be at the peak of his powers, but there was a time, he says, 'when I went through a crisis; you know: 'What is an actor, what am I doing?' '
In the process of answering those questions, Suchet worked out the method which underpins his professionalism. 'My work has always been to become different people,' he explains. 'And in becoming a different person, with all their vulnerabilities, you can either pretend and be unaffected by what you do -
and there are many famous and respected actors who do that - or you can experience what that person feels. The latter is what I do. I'm not saying it's better or worse. But that's the only way I can be.'
'What a Performance': now previewing, opens Wed, Queen's, W1 (071-494 5040).