So when you learn that the play she's rehearsing at Hampstead focuses on two sisters, one of whom is a loud Florida matron with a lust for life and a degree in 'cosmetology' (that's face make-up to you and me), the mouth starts watering for another Steadman special. But wait. Steadman is playing the other sister in Scott McPherson's Marvin's Room: a self- effacing soul who has spent her life caring for her bedridden father.
'She's a middle-aged spinster, a shy sort of person,' says Steadman. Sitting in the Hampstead Theatre conservatory, she looks different from usual. It's the hair, she says. 'The director suggested I had it dyed for the part - it's usually blonde. I think blonde can equal glamorous or outgoing.' She admits that, on reading the script, she too saw the louder sister, Lee, as her natural territory.
'But, having done Mari Hoff in Little Voice, I thought it would be more of a challenge to play someone who wears dowdy sandals and plain frocks and doesn't think of herself as attractive. It's true that the sharper the character, the more fun they are to play - there is less potential for comic stuff in this part. But in the end something drew me to it. Because I've done those parts, you know, your Beverlys and your Mari Hoffs.'
It is characteristic of Steadman to try something new. Most of her work has been with new scripts, and she has appeared in several films and plays by her husband, the writer-director Mike Leigh, who famously starts with an idea and a cast and writes his scripts on the hoof, building on the improvisation work done with the actors in rehearsal. You can't get much fresher than being in on the creation of a part, and Steadman admits to missing that in Marvin's Room, which has already been a big hit in America, with an American cast . 'To be honest, I'd rather that I was the first person ever to play this part, but you can't do anything about that. I don't want to see the photos or read the reviews - I want it to be mine]'
Most people would find the thought of improvisation alarming, imagining it easier to have a script in your hand. Not Steadman. 'I would say the written word is more worrying. If you're doing a film or a play with Mike, you're nervous but you don't know what you're nervous of, because you don't know what's going to happen. But with a script you know what you've got to tackle in the next four weeks. You've got to make it your own.
'Also you have this sense that you've got to serve the play and the playwright. I remember being in an Alan Bennett play at the Royal Court and he came to watch a rehearsal. I was beside myself with nerves, because there can't be anything worse than writing a part and then watching someone doing it all wrong.'
Alison Steadman is 46. Her acting career began in her hometown of Liverpool. She was the class mimic - 'I joked my way through school' - and she joined the Liverpool Youth Theatre. She says she was set on being an actress from the age of nine. Her parents, realising she was in earnest, didn't object, and she went on to the E15 drama school, which encouraged a valuable openness and with it an interest in freshly minted parts.
'In 1966 Rada was still quite rigid. It was still giving out these health and beauty prizes or whatever - it was still a little bit of a finishing school. E15 was kind of wacky and wild - it suited me down to the ground. It wasn't as organised as it should have been, but it gave everyone a good grounding, so you stopped thinking about yourself and started thinking about what your function was in the play.'
While performing at Liverpool's Everyman Theatre, Steadman met Mike Leigh. They have been married for 20 years, and their marriage and working relationship has long been the source of fascination and speculation. Does she tire of this interest? 'Yes,' she says, without any hesitation. Then she relents. 'I realise it's what people are most interested in. I do get a bit fed up being asked, 'Don't you ever get bored with each other?' If that was the case, we wouldn't work with each other. I mean, that's how we fell for each other in the first place - we have this creative rapport.'
Leigh recently won the Best Director prize at the Cannes Film Festival for Naked. The award, the latest of many they have won between them, is unlikely to affect life at home in Muswell Hill for Steadman, Leigh and their two sons. 'To them it's just ordinary if I'm on telly. Their main fear is that they'll be embarrassed by something I do.'
She chats away unpretentiously, just as ready to talk about her addiction to shopping as her approach to a part. The Liverpudlian accent still lurks and comes shrieking out when she's carried away. She tells some good anecdotes about filming picnic scenes on Brighton beach in thermal underwear and moon boots in thermal underwear and moon boots, but she prefers the stage to film, finding the creative process in theatre to be harder, but more rewarding.
'For Little Voice, rehearsals were sometimes hellish because it was like wrestling with a tiger, trying to pin it down. But when you finally did get it, and you were out there in front of an audience, it was like you were 20ft high. I always think it's a bit like having a baby - if you didn't have any pain when you were giving birth, the baby wouldn't be that precious. I mean, if you dropped him on the floor, you'd think, 'Oh well, doesn't matter, I'll have another one.' The day you walk into a rehearsal and say, 'I can do it', you might as well not bother, really.'
Steadman's bent towards improvisation seems to reflect her attitude to her career in general. 'I never know what I want to do until it turns up on the doorstep. I've never planned anything.' But there are those who feel she has not had her share of classical roles. Does this frustrate her?
'I don't know really,' she says, with a shrug. 'I have done some Shakespeare, but if you tackle one of those parts you do always feel that A N Other has tackled it before, and I don't like that - I like the freedom to just do it . . . But I wouldn't rule it out,' she adds, hastily, giving the tape-recorder a mischievous glance.
'Marvin's Room' is previewing now, opens Tues at Hampstead Theatre, London NW3 (071-722 9301).