It's a part that Graves seems absurdly right for. There is his trademark little-boy-lost appearance (an appearance that seems unsullied by the decade that has passed since the handful of Merchant-Ivory dramas that first brought him to attention: A Room with a View, Maurice and A Handful of Dust). Those smoulderingly sheepish looks, so irresistible to women, have Julie Walters' prim and improper surrogate-mum Marjorie offering him much more than just lunch-box sandwiches to keep him going during his stay. There is his physical presence, too - that game-for-anything sporting agility that exposes the post-war suburban souls he lands up with after years at sea as the grotesque embodiments of their society's overly rigid moral order. Above all, though, there is Graves's intrinsic enigma. That blank-canvas quality which allows the film to rise above the status of mere dramatic reconstruction (who should we blame?) to become a blackly-comic portrait of clumsily articulated desire (who isn't to blame?).
Like Guppy, Graves dares you to pin him down. For ages, he pick 'n' mixes his way through a variety of accents: anything from a spoof of his native west Country (he's a Weston-Super-Mare man) to a broad American. "It's nerves," he insists. "I'm never sure whether I've made a mistake in the film by trying to make Guppy too nice," he says at one point but generally he keeps his comments about this passive yet impulsive character to a minimum. Talking about the sex scenes poses few problems however: "I always think that any scene which you don't have to speak in, like running across a field or shagging is a relief because you don't have to concentrate too hard."
For a long while, Graves's enigma went hand-in-hand with his preparedness to expose himself in the bedroom. In the televised drama of the Steven Waldorf killings he acted the transvestite part, while in the EM Forster adpatation, Maurice, he snogged actor James Wilby. There was also the West End version of A Torch Song Trilogy where he played Antony Sher's boyfriend and the Martin Sherman play, A Mad House in Goa, where he had to simulate more naughty things with a man under a duvet.
For a long time, he admits he enjoyed playing guessing games with journalists who enquired about his sexuality. The truth is heterosexual domestic bliss with mature student, Yvonne - and greyhound, Roland - in Stoke Newington. As it has been for 10 years. "I used to say it really was nobody's business. I could have said, `No, I'm not' but I chose not to. I do have a worry that Al Pacino has. He says that if you do interviews and chat shows, you're asking the audience to suspend their belief in you as someone else. I like the idea of being anonymous. I talk to the old biddies in the park when I'm with Roland and I'm not recognised very often."
There was perhaps a similar confusion about his class background when he first came to London, after his professional debut at the King's Head. There were, apparently, assumptions that, with his public-school looks, he might even be related to the poet Robert Graves. Hardly. He had just finished a stint wrapping chips in Weston's Taste of Fish. With only one O-level to his name, Graves hasn't got good memories of his alma mater, Wyvern Comp. "It was all right for sleep. I didn't enjoy it very much because it was boring and I had glandular fever so I missed a lot. I was a dozy boy, I'd like to have been like James Dean but I was more Arthur Askey - pathetically rebellious in a cheeky chappy sort of way."
By 16, he had made one appearance in the school production of A Twelfth Night and had sung his way through three gigs with his pop band, A New Lumbago, at Weston Tech. "I kind of always wanted to act but to get a grant I would have needed two A-levels and I was too far away from even O levels. I didn't know you could get a scholarship so I determined early not to pursue that." So when the circus came to town, courtesy of the YTS, he joined up as a clown. Primarily, so he reckons now, because he had been in love with Juliet Griffiths since he was seven and her mum "worked down the job centre". "I was employed mainly to put the tent up because there were very few able-bodied men. But I did learn juggling and slack wire."
At only 22, he landed the role of Helena Bonham Carter's younger brother, Freddie, in A Room with a View. If the parties were a culture shock, so too were the London thesps. "I didn't have a clue what anybody was talking about. People were referring to writers - Ibsenesque and Chekhovian - and I thought, `What are you talking about?' Chekhov - oh right, does that mean quieter?" Did anyone ever take the mickey? "If they did I was probably too green to notice. Or it was too subtle for me."
An early role in `Tis Pity She's a Whore at the National is still etched in his memory as an evening of unmitigated horror. "I had to deliver this speech after I came down a tunnel and on the first night I forgot my lines entirely. I started sweating, remembered the first line and then just shrieked. I wanted to say to the audience, `Come back in about five years and I'll probably have got there.' I thought. `This wouldn't have happened, Graves, if you'd gone to drama school.' "
He's now fully reconciled to missing out on drama school but reads furiously because he still feels so "uneducated". Currently he's reached page 40 of War and Peace. "Even now when people say, `Read this or that', I think I've never heard of them." Does he let on? "Depends on the company. If I gauge them as sympathetic I will."
Success has never been guaranteed despite, or perhaps because of that early exposure. Still, at the very least, he can make a living from his acting, not something that can be said of many actors. He claims he has only recently been able to contemplate swapping renting for buying. Several years ago, finances were so dodgy that the bailiffs were making regular appearances on his doorstep. "It all caught up with me. I'd given some money to an accountant who scarpered and then I had a friend in Weston who was doing investments and I lost all that."
Cash, though, does not motivate him, he says. "I've done films for money like The Innocent Sleep and an Italian film which was shite and a film in Namibia which was shite. But although I get little flashes of panic, it's not enough to think I have to do big, expensive movies."
Later this year, he co-stars with Vanessa Redgrave in the screen adaptation of the Virginia Woolf novel Mrs Dalloway and in November he'll be seen in Bent. He's written two short stories himself that he'd like to make into two films but is not sure he'll ever get around to letting anyone read them.
"My grammar's crap - it's almost embarrassing for me to read it, let alone anyone else." He knocks back another Stella. "I still feel like a Weston boy. I'm not very intellectually gifted and even now I can't say `miles' properly."
But after four lagers, he does do the best impersonation of a greyhound I've ever seen.
`Intimate Relations' is on general release