Ruth Wilson on her sizzling summer with Jude Law

You can see why they call Ruth Wilson a 'dame in waiting'. She is the real deal: intense, truthful and luminous on stage and screen in roles such as Jane Eyre or Queenie in the BBC's

In fact, she recently did exactly that, in a little escapade that began as a cast-bonding exercise. The second and third acts of Anna Christie are set on a barge, and the sea is constantly tossing the characters about, controlling their destinies. For a taste of shipboard life and the power of the sea, Ruth thought she would invite 'the boys' - David Hayman, who plays her father, and Jude Law, who plays a sailor she saves from drowning - to come and spend the weekend aboard an old-fashioned fishing boat, which her family owns, on the East Coast. 'It's a pretty basic boat, you know. It's not a luxury yacht. I didn't expect Jude to say yes, but he was very keen and mucked in completely. He instantly disarms you because he's so generous and open and hard-working. There's no ego at all, with Jude.' Which was a good thing because all hands were needed on deck as, somewhere off the Norfolk coast, the stormy sea started blowing a Force 6 gale. 'It was rough, but it was exhilarating. It's exciting, isn't it? A force of nature you can't control.'

That's the spirit of Anna Christie, a magnificent, daunting role previously defined by Greta Garbo in 1930 (her first non-silent film, occasioning the strapline 'Garbo Talks!'), and, on Broadway, by Liv Ullmann in 1977, then Natasha Richardson in 1993, opposite Liam Neeson (they fell in love during the course of the play and married soon after). The chemistry between Anna Christie and the sailor Matt Burke is a key factor in any production. 'Sure,' says Rob Ashford, who is directing this Donmar version. 'It's week three of rehearsals, and it's all there.' He knows Ruth well, having directed her in 2009 in A Streetcar Named Desire at the Donmar as Stella, a role often overshadowed by Blanche DuBois, but one in which she shone, winning an Olivier for Best Supporting Actress (Rachel Weisz, as Blanche, won Best Actress). 'I didn't properly understand Stella until Ruth auditioned,' he says. 'I loved her honesty and truthfulness in that part. Ruth is so strong and feminine. Without even trying, she's kind of bathed in sensuality. It's in the way she moves, the way she sits - she's not sexual, she's sensual. It's palpable, that pulse is always beating.'

Ruth plays her cards close. 'I'm not going to say,' she smiles enigmatically when asked if she has a partner. There is nothing overt, nothing girlish about her sensuality. But it is definitely there. 'Masculine tailoring I find quite sexy, actually,' she says over lunch. 'I like structured, tailored clothes. A well-cut suit makes me feel strong and, strangely, more feminine.' Has she ever worn tails to a party, Dietrich-style? 'No, but a friend of mine, the tailor Adrien Sauvage, has designed me a whole range of double-breasted suits. So maybe I will.' One of her eyebrows gives the smallest, subtlest kink. Wow. That's what set Mr Rochester on fire.

'I've always been a tomgirl. I mean, a tomboy. Perhaps it's to do with having three older brothers. I'm not comfortable being overtly feminine in a tits-and-ass sort of way. I don't really like that kind of attention. I'd rather connect with people on a level which is more than that. Which is about having a conversation, not "look at my bum and look at my legs". Sometimes it makes you feel empowered, but most of the time I don't like doing that. I never grew up using that approach. It makes me feel quite uncomfortable.' She is, after all, a nice convent girl. She grew up in Shepperton and attended Notre Dame School and Esher College. Degrees at Nottingham (in history, though she 'felt quite alienated by academia' and mainly focused on acting) and LAMDA followed.

She has rarely taken what she calls 'eye-candy' parts - 'Well, to be honest they don't often come my way' - preferring strong female leads: in the BBC detective show Luther her character runs psychological rings around Idris Elba and industry speculation has her down for a lead in the forthcoming Disney Lone Ranger film alongside Johnny Depp. She believes that 'although people think the feminist work has been done, it's not over, it's a constant battle that needs to continue'. She thinks more women writers and directors will result in better roles - and role models - for women. She was in talks to produce, with Emma Thompson, a female-centric film project, until the UK Film Council closed and money got 'very tight'. 'It's so important to create female characters that young girls can respond to. I am amazed by the letters I get from all over the globe about Jane Eyre.'

There is a strong history of military service in the Wilson family, with one uncle a former captain in the navy, and one brother serving in the Territorial Army in Afghanistan. 'We were taken to church by my dad every week. Being Catholic was a huge part of my childhood. I don't go any more. Like most people, I think, I have quite a conflicted relationship with faith and God. I'm fascinated by religion, and that's why the last few pieces of work I've done in the theatre have been about that - such as Through A Glass Darkly [in which she starred at the Almeida last year, as a woman in spiritual crisis]. Anna Christie finds redemption through the sea and the almost Jesus-like character of Matt Burke I've got lots of friends who are very strong in their faith and it always amazes me, the strength and safety net they have in their lives, and their con-fidence that everything happens for a reason. I'm very envious of that, but I can't quite go there myself.'

Does she think she will go there, perhaps later in life? 'Sometimes I think that in order to develop very strong faith, you have to have a moment of despair or something in your life that makes you need it.' So: hopefully not, I suggest. She laughs. 'Well, I'm sure I'll have a bit of despair, I've certainly had my moments, though I haven't found God yet'

Others close to her have, however. Her family was nearly torn apart in 1963 when her grandfather, Alexander Wilson, died of a heart attack - and turned out to have been very different from the man everyone thought he was. Her grandmother Alison discovered papers that indicated he had another wife and family, and later researches revealed two further marriages, both with sons. His extraordinarily rackety life included espionage for MI6, spells in prison and authorship of thriller novels based on his experiences (such as The Devil's Cocktail, published in 1928) and his story has been told in a book, The Secret Lives of a Secret Agent. Ruth describes the impact of these revelations on her grandmother. 'My granny had an amazing religious experience and dedicated the rest of her life to God. She became almost like a Bride of Christ, although we didn't know any of this at the time. She was betrayed on a huge level and the only person she could go to who she knew wouldn't betray her in the same way, wouldn't hurt her, was God.'

Ruth is speaking fast and fluently now. She's at home talking about the big stuff. 'She could play Hamlet, no question,' says director Rob Ashford, when I suggest it to him. 'She has the depth for that.' In preparing for her roles, she has developed an exacting method. She flew to New Orleans before playing Stella in Streetcar, and recorded local people talking, soaking up the atmosphere. For Anna Christie she went to Minnesota, photographing buildings ('So when I talk about a farm, for example, I have a visual image of that farm in my mind') and speaking to local history professors. 'I much prefer talking to people than getting it from books. I like tapping into information that way, getting to know people with different interests to my own.'