Alfred Kinsey, the famous sex researcher, had a rule for those who conducted his intimate questionnaires. Don't ask: "Have you ever had sex with a goat?" he instructed. Always say instead: "When was the last time you had sex with a goat?" Or: "How often do you have sex with goats?" These eliminated the element of shame, he explained, and allowed the interviewee to assume anything might be considered normal.
Journalists tend to frame questions, unwittingly, in ways that can be off-putting to the interviewee - but I've never had an opportunity to ask Kinsey's exact question outright. Now, here before me, is Damian Lewis, currently in rehearsals for Edward Albee's The Goat, or Who is Sylvia? - a play about a man who confesses to his wife that he has fallen in love with a goat.
"Very good," he says with an approving but unemotional nod, as if this were a game of chess. "Are you going to ask me the same question?"
I nod back. He takes a mouthful of salad. Swallows it.
"My research hasn't taken me that far yet," he says. "But."
Another mouthful. "We have found out some surprising facts."
Ian Rickson is directing this production, which sees Damian back in his hometown of London, and the way he likes to work involves inviting various experts to come and talk to the cast. In this first week round of rehearsals, they've had a classicist tell them about the traditional structure of the play, as well as about bestiality through the ages; and they've heard a psychotherapist tease out the psychoanalytic strands. In the process, Lewis asserts, they've discovered "a high - I think surprisingly high - percentage of people have had the fantasy of sex with an animal. 10pc."
That's just the fantasy. But, he adds, "we had a woman come in the other day to talk to us, and she said that it still went on - here in this country, isolated farms, men and sheep…"
"What kind of expert was she?" I ask.
"Er, she wasn't an expert," Lewis admits. "She was just a friend of the cast." Lewis is dressed up - or at least he seems that way to me. He has entered the restaurant where we meet for lunch wearing a urine-coloured leather coat, pilfered, he says, from a theatre production over a decade ago. He flings it on the seat beside him, revealing several more layers beneath: a blue cotton jacket, a tweedy waistcoat, a button-down shirt and loosened tie. His pale red hair is whooshed back, as if styled in a wind tunnel. The ladies lunching around us turn to look at him and whisper, and despite Lewis's low-key, always affable manner, he hardly appears as if he'd want it any other way.
He explains that it's two years almost to the day since he started rehearsals for his last stage performance - David Mamet's American Buffalo. He and his co-stars John Goodman and Tom Sturridge always had lunch in this restaurant, and they always ordered the same salad, which seems to include most of the ingredients in the kitchen.
"Just a minute," Lewis says, getting out his iPhone. "I have to email a photo of the menu to John and Tom to let them know I'm here."
Lewis is 46. Though often mentioned in the same breath as younger fellow public school-educated British actors Eddie Redmayne and Benedict Cumberbatch, his career has been markedly different from theirs. He has predominantly focussed on serious theatre and smart, high-class television, both of which are more intimate endeavours than major Hollywood films.
Lewis's stardom - which began in 2001 with Band of Brothers, the wartime series produced by Steven Spielberg and Tom Hanks, and exploded with Homeland, in which he played Nicholas Brody, the US Marine turned possible Al-Qaeda killer - has brought him into our living rooms, week after week, and left us hanging on the edges of cliffs. He is commanding but ambiguous.
If there is a unifying thread - and this goes some way towards explaining why viewers have loved his many complex roles so much - it's that Lewis trades in heroism, taking stirring ideas about loyalty and showing all the creases in them. Spielberg cast him as Dick Winters because he'd seen him on stage as Laertes in Hamlet, a pained role of double revenge. In Wolf Hall, the series based on Hilary Mantel's prize-winning novels, he turned Henry VIII into a very human sort of king. Now his steely gaze is evolving in Billions, a New York financial thriller in which he plays the slick hedge-fund manager Bobby Axelrod.
Lewis tells me that he always wants "a job to be serious in some way". He knows he has to be careful not to offend anyone when speaking of the work that sustains him, but that's what he's looking for - and just now, that means the stage.
"I find that theatre, increasingly, has started to attain a spiritual dimension for me in my life," he says. "I do think that theatre, at its best, has a shamanic quality. And I think the collective consciousness is very strong in a theatre. That's because of the moment of shared experience - the moment at which the story is told for the first time that day, or new-minted that night by the cast: as it is spoken, it is received by the audience."
That isn't true of film, he explains. "So I do find that there is something congregational about theatre." And already, just in rehearsal, he says, "I feel replenished by it."
US playwright Edward Albee's play, written in 2000, is a transgressive, layered thing: shocking, and perhaps absurd. Yet it explores, by the apparently improbable means of an affair with an animal, universal ideas about love, and tolerance.
"It was at that moment that I realised… that she and I were going to go to bed together," says Lewis's character, Martin. And, to his wife: "I won't go into the specifics of our sex with you!"
These lines do not suggest that the "other woman" is a goat. A not-quite-subplot is Martin's relationship with his gay son. How audiences in London's West End will respond to the play remains to be seen. Albee's work is, in Lewis's description, a "mind-bending play".
One of the things the play has made Lewis think about is "something that's always uppermost in my mind: how present you are as a father for your kids. And how important it is to be present, and how quickly you feel guilt when you're not present..."
Lewis is married to the actress Helen McCrory; they have two children, aged 10 and nine. As far as anyone can tell, they manage to balance two very successful careers with a relatively sane home life in London. It doesn't always go exactly as planned.
Last year they meant to move to New York as a family, while Lewis made Billions. But McCrory, who had recently performed both Medea and Terence Rattigan's The Deep Blue Sea at London's National Theatre, was offered the leading role in Fearless, a six-part ITV drama scripted by one of the writers of Homeland. They all agreed, Lewis says, that she should take it.
"We were trying to work out how we were going to remain: a) together; b) happy; c)…." He drifts off.
"What's c)?" I ask.
"God knows," he says, laughing.
"A) together and b) happy - I think that's enough, for one indiscreet interview."
Lewis ended up taking the children with him to New York, alone.
"I'm not going to pretend that I swanned through it - I felt pretty frazzled by the end of it," he admits. "I think Helen adjusted… alarmingly well, shall we say, to four months without the kids or me. Never seen her happier, calmer."
In this mid-life moment, he says, he craves sleep, and time to read, and… "I'd say 'stillness', but I'd be lying," he reflects. "It's a constant duality, isn't it - trying to elasticise time: wanting stillness, but actually being a junkie for the excitement of life." Sometimes, he says, life seems like "a series of incomplete ideas, actions, thoughts..." He laughs. It's like, he says, "you're in a Laurel and Hardy sketch and you didn't get out of the way - Stan turned around with the ladder and you got hit on the head, and then you stood up and it comes and hits you from the other side. I feel that's what life's like."
Lewis doesn't think his epiphany, if it ever comes, "will involve Cupid's arrow and a goat". But you never know.