Your latest role, in BBC One’s Born Equal, is a mother living in a hostel to escape abuse. Sounds melancholy.
Actually, it’s a relief to be acting about real life. Too often acting feels frivolous, or superfluous.

A big contrast with your much-loved Fiona in Shameless?
Shamelessness was part of its attraction. Fiona, was so fully formed; the linchpin of her family, and sexy and flirtatious. She was sometimes immoral but had a strong morality.

Howard Davies, the director, once said you throw yourself at parts as if bruising yourself on them.
Tough roles should bruise. I’m not frightened of emotional mess like other actors aren’t frightened of sliding down banisters for huge laughs. It depends which skin you are strongest in. Somehow I find it more appealing bruised.

Sounds pretty heavy.
If you saw me in my kitchen you’d realise what a giddy kipper I am. Who knows what weird and wonderful direction I may end up taking?

Like slipping into Queen Elizabeth’s hooped petticoats for The Virgin Queen?
Oh my God, that was hard graft. I had worked on period dramas before but only by way of melodramatic bosom-heaving in corsets.

Any qualms with your body-obsessed profession?
Yeah, I find this profile-building anorexia — ie, the slimmer you are the more success you will have — absolutely petrifying. I find it dreadful that we are fed these weirdly glamourised, unhealthy images of stick-thin celebrities in magazines.

What about the ageing celebrity debate?
It’s a wee worry as parts are harder to find for older women. But the sexiest actors I can think of are older women such as Charlotte Rampling, Helen Mirren and Juliet Binoche. I never won a role with my looks anyway, but it must be terrifying for a ravishing beauty when age begins to catch up with her.

Would you ever go under the surgeon’s knife?
Doesn’t appeal to me in the slightest. Plastic surgery doesn’t make you look younger, it just looks like plastic surgery. And it’s weird seeing a pair of 60-year-old eyes in a younger face.

Got a bit of a taste for green tea, then?
More of an interest, but probably only because I have never had an acute illness.

What interest; supplements to beat the winter chills?
Vitamins, healthy eating, a good massage and laughing your head off once a day all do no end of good. If all else fails, my mother’s got a great cuddle and that’s a perfect medicine.

Ever need to unleash some nervous energy?
I can be a bit of a worrier.

How do you keep on top of it?
I have learnt to control it by just telling myself to shut up.

Other than your boyfriend, the actor James McAvoy, what gets your heart pumping?
Visiting my gym when I’m at home in London. To be honest, I don’t always like it, but it’s great when I’m uptight; I pull on the kit and really get the blood pumping; grrrr!

Speaking of hearts, you’ve said yours needs old-fashioned care. How so?
I protect and respect my heart. It’s not up for public consumption. My hopes, fears and feelings for other people are precious. When people reveal themselves, they give away pieces of themselves that they shouldn’t. I prefer old-fashioned mystique. Apart from anything else, I don’t want audiences to bring what they know of my private life with them when I perform. They need to be lost in my storytelling.

How do you get your kicks?
My God! I thought you asked something entirely less appropriate. I was all ready to indignantly beg your pardon and say that these are 100 per cent mine!

Breasts aside . . .
I get my kicks hiking, climbing, hill-walking and camping. I love, love, love getting away from the city, standing exhausted on a hilltop and just looking. It’s my favourite thing, usually laughing my head off with someone and getting to be a big eejit kid again. Also, burnt sausages never taste as good as they do under open skies.

Is your glass generally half-full or half-empty?
So full that some say my sanguine personality is quite annoying. I’m a right Pollyanna.

Speaking of full or empty glasses, what’s your poison?
I get very, very, very drunk very, very, very quickly. After which embarrassing admission, this will sound strange; I love a wee dram of whisky, my favourite being a single malt called Tobermory from the Isle of Mull. Better make it just the one, though!

Greatest fears, ultimate ambitions?
Without the love of my family and friends I would be bereft. My ambition is for people to keep asking me to do their jobs. I’m a junkie for my work, such a lucky girl, you know. I had a year when I didn’t work and it was tough not to go mad. But that risk appeals to the daredevil gambler in us actors. Sometimes acting is like standing at this huge gaming table, a real thrill-a-minute thing!

Born Equal, BBC One, December 17, 9pm

Times recommends

• Jamie Oliver's boiling over
• Family homes for the 21st century
• What makes men commit?