The casting director, Gail Stevens, isn't worried. 'Go for the character; go for quality,' she says, and, although this is what you would expect her to say, she is right. George Eliot likes Will Ladislaw (half-English, half-Polish, ed. Rugby and Heidelberg) so well that she smothers him; even fans say that Will never came to life. Sewell, who began to smoulder in Wednesday's episode, is breathing the life back. At 25, Sewell is the latest 'juve' - the dashing juvenile lead, like Jeremy Irons, Kenneth Branagh and Daniel Day-Lewis before him.
When I spoke to Sewell last weekend, he hadn't seen the reviews for Middlemarch. He was in Los Angeles, seeing agents (he left before the earthquake). But he was not surprised to hear that they had been so good. When filming began, he tried not to have high hopes: 'I told myself it would probably fail. But after a while I couldn't say that any more.'
Gail Stevens backed a hunch when she put Sewell's name at the top of her list. His agent had sent him to see her after he left drama school, and she had been struck by his looks. 'He has a slightly wild quality that makes him unusual,' Stevens says. But she missed him at the Compass Theatre in The Royal Hunt of the Sun and at the Royal Exchange, Manchester, where he was cast in the quintessential costume part of Mr Darcy in Pride and Prejudice. (Sewell does not always wear fancy dress: in his film debut in Don Boyd's 21 he was a 1990s drug addict, and for his appearance in the TV series Gone to Seed he wore nothing at all.)
Stevens first saw him in his West End debut, in James Saunders's Making It
Better, playing a Czech student who seduces Larry Lamb. Louis Marks, the producer of Middlemarch, had seen him too: 'I liked the non-English looks, the suggestion of the Polish.' And so had the Press: he was the Critics Circle's Most Promising Newcomer of 1992.
But Tom Stoppard had also seen Making It Better and decided that Sewell would suit the part of the tutor in his new play, Arcadia, due to open at the National Theatre in the spring of 1993. The best day of Sewell's life so far was early in December 1992, when he was offered both parts. Arcadia was offered and accepted first. 'At one ghastly moment it looked as though we couldn't have him,' Gail Stevens says. At times he was due to be filming in Rome and rehearsing in London on the same day, but enterprising production managers managed to mesh his two schedules.
Stoppard had written a fine part, combining intelligence, authority and sexuality; to it Sewell brought clarity, energy and his good looks. Far from being intimidated by a cast including Felicity Kendall, Bill Nighy and Harriet Walter, he stole the honours, confirming his rich promise as a young actor. In Middlemarch the charm and sexuality have already been impressive. Sewell thinks the series gets better after the third episode (this Wednesday). If so, it ought to make him famous.
He is insouciant about doing two things at once. 'Basically, the technique is like going from a maths lesson to an English lesson,' he has said. The metaphor would amaze his family and his teachers. Middlemarch is only one of a long list of books that Sewell did not read at school. He only read it when he was told about the part in it for him. 'I took it to the theatre when I was in Making It Better, and I read the first page 18 times,' he says. 'I didn't get into it until I took it home and stayed up two nights in a row.'
He grew up in Twickenham, with his mother, who is Welsh and separated from his father, an Australian animator. They let him develop in his own way - which involved bunking off school regularly. When it came to O-levels, he scraped six, just enough to get a place in the Central School of Speech and Drama. There he continued to behave in a mildly anarchic way, sleeping in a loft above the stage and returning home only when a wash could be put off no longer.
A hint of this anarchic nature is still there, along with real charm, fierce ambition, and a refreshing kind of realism. 'I'm not suddenly that much better than I was last year,' he says disarmingly. And Sewell, currently the companion of Helen McCrory, a fine young actress, scorns the idea that they might form a theatrical partnership to rival that of Kenneth Branagh and Emma Thompson. 'It's not hush-hush, but I don't want to talk about it. To become known as a couple is not healthy for either of us.'
Sewell has just turned down a big film offer (to spare the blushes of the actor who accepted the part, he will not identify it). Instead, he's taken another costume part - Merton, the journalist, in a film of Henry James's The Wings of a Dove. A former colleague at the National Theatre says: 'I'll be interested in seeing him in a part where his looks don't remotely count. I mean, what will happen when he has his hair cut? But he seems very level-headed indeed. I suspect he'll become mega.'
'Middlemarch': BBC1 tomorrow (episode two, rpt), BBC2 Wed (episode three).