Romola Garai talks fast and thinks faster. If there is no actual steam coming out of her ears, the high-pressure stream of her enthuasiasms is there for all to see. “I am passionate,” she says with a big ‘so, shoot me’ smile. “I’m passionate about everything - people, art, politics, life in general.”

It’s not, she goes on, a quality that sits easily with Englishness. “People find passion embarrassing,” she says. “It’s more acceptable, generally, to be amusing.”

Garai is, in every sense, a serious actress; not earnest - she laughs a lot, often at herself - but deeply thoughtful about her profession and its responsibilities.

This week sees her return as Bel, the prototype career woman in a new series of The Hour. Set in Broadcasting House in the 1950s, the drama, with its winning formula of spies and seamed stockings, was originally hailed as Britain’s answer to Mad Men, but quickly deepened into an astute political thriller.

Garai, who can rock a pencil skirt with the best of them, is careful not to overstate the case for Bel as social pioneer.

“Bel is a sort of fantasy. There’s no way she would have been in that position - producing her own show - at my age [Garai is 30]. She would have been there for a very long time and in a very lowly position before she was given that kind of opportunity. But it’s a drama and Bel’s a great character. So I’m not complaining.”

The Hour, with its murky undercurrents - the new series takes a detour into racial violence and gangland intrigues - is an effective antidote to the cosy, cupcake version of the 1950s currently very much in vogue.

“I find it strange when women get nostalgic for that era. I can see just about that we have lost some of what might be called the security of being in the home, but what we’ve gained seems to me so much greater.”

The fact that so many of her own generation now disown or dislike the term feminism is, she says, “bewildering”.

“I’m a feminist,” she declares, “God, yes! A bra-burning, building-burning feminist. I was brought up with a very strong sense of what can happen if your society starts to chip away at the small victories women have won for themselves. I remember when I was about nine, there was a timeline of British history on the wall at school and votes for women was about an inch before the end. We’re just a hundred years into having any history of our own and I never forget that.”

Educated at Stonar, an independent school for girls in Wiltshire, Garai - the daughter of a journalist and a banker - was a year into her English degree at the University of London when she was offered a part in the film The Last of the Blonde Bombshells.

“I’d already been involved in drama on a miniscule scale and this seemed too good a chance to miss. So instead of going back to university, I just kept plugging away with the acting.”

The plugging soon gathered momentum. Starring roles in Daniel Deronda, Emma, Atonement and The Crimson Petal and the White earned critical praise for Garai’s nuanced, truthful style. In the meantime she earned her degree - first-class honours - with the Open University. Lacking formal dramatic training, she relies instead on observation and empathy.

“The really brilliant actors I know, the transformative actors, are endlessly interested in other people. I think that’s what drama is for. Stories - whatever the subject - are important because they teach you to empathise with the feelings of other people. It’s a civilising force.”

Less civilising aspects of showbusiness are met with less empathy. In particular, Garai is concerned about the effect of body dysmorphia on both actors and audiences.

“My weight was a very big issue when I started,” she says. It’s not a conversation most young actresses would choose to start and given her enviable figure - a double whammy of long limbs and curves - seems incredible. But this, Garai insists, is the point.

“Yup. It’s true,” she says. “I was then - and am now - a very normal size ten. But that’s not acceptable. Everyone’s aware of it. It’s partly because fashion, film and television have become so interdependent. Increasingly, it’s actresses doing the big fashion advertising campaigns and now there’s no distinction between actresses and models. There’s no way I could ring up a company that was lending me a red-carpet dress and say ‘do you have it in a 10?’ Because all the press samples are an 8 - I would say a ‘small 8’. If you want the profile, you have to lose the weight.”

“It’s difficult because if I refuse to do any magazines at all, my work, I think, would suffer in a very immediate way. But when I appear in these magazines, I know I’m being ‘trimmed’, I’m being airbrushed a lot. And I know that people are accepting those images and are under the impression that that is really how my body looks, that I’m hairless and sexless and weigh 90lbs. That really worries me. And I really don’t know what to do, except talk about it.”

An acknowledgement of style over substance - as in the literal substance of female flesh - would, she thinks, be a start. “Look at Tilda Swinton. I think she’s incredibly beautiful, immensely stylish. And she also doesn’t give a damn. Which is the best possible thing.”

She may be right, but it would be a shame if Garai were to stop caring. Because she does it so well. Not just in an ‘isn’t it awful?’ way but in a considered, career-defining way. The lure of Hollywood, she insists, is considerably lessened by the attitudes of US film executives.

“I was just reading the other day that Jason Segel was made to lose 30lbs to play his part in The Five-Year Engagement. Because the executives said it just wasn’t credible that anyone would want to have sex with him the way he was. I think that is such a profound misreading of what people want out of sex and relationships. And I want no part of that. I wouldn’t want to sit in a room and have someone say to my face, ‘No one is going to want to have sex with you.’ No job is worth that.”

Meanwhile, there are the “piles and piles” of books waiting to be read in her west-London flat and a Guinness cake recipe she’s keen to perfect. There’s her new movie, Last Days on Mars, co-starring Liev Schreiber, to be shot in Jordan, in a spacesuit. “I know the director’s going to be saying ‘How do you want to play this scene?’ And I’ll be saying, ‘Well, really, I’m just going to try not to faint’.”

There’s also the drama, Scrubber, which Garai has written and directed and which was nominated in the international short film category at the Edinburgh film festival in June.

“Obviously,” she says, with no shred of irony, “I’m coming to writing and directing a bit late, but I’ve really loved doing it.”

So many passions, so little time. If Hollywood sharpens up its act, she may give it a whirl. If not, she won’t be sitting by the phone. But the question isn’t ‘Will Romola Garai be big in movies?’ More to the point, will the movies be big enough for Garai?

The Hour returns to BBC2 tonight at 9:00pm