“I turned up in jeans and a hoodie and everyone else was really cool, really trendy,” she remembers. “Nobody was over a size ten. I stuck out like a sore thumb.”

“When your role comes up, you’ll know,” the casting director consoled her. “People just have to wait for the right role.”

At 24, Rooney has had to wait longer than most — and she’s under no illusions why. “There aren’t enough scripts out there for people who don’t conform to the stereotype,” she sighs in her gentle Glaswegian accent.

“It would be so nice to see, not necessarily just bigger people, but normal-looking people.”

In My Mad Fat Diary, Rooney plays Rae, a teenager who is as hilariously, heart-wrenchingly candid as Adrian Mole. “Dear Diary,” Rae says matter-of-factly in the first episode of the six-part series, “I’m 16, I weigh 16 and a half stone, and I live in Lincolnshire.” She also has an unfortunate habit of self-mutilation and has spent several months in a psychiatric hospital.

“I wish so much there had been a Rae when I was growing up. It would have made my life so much easier to have had someone real on TV that I could have looked at and gone: ‘I kind of look like her. I don’t look perfect, but she’s got friends. People love her so maybe people will like me for being me. I don’t have to change. I can just be myself...’ How can kids and teenagers feel comfortable when they can’t see anyone who looks like them anywhere?”

The real wonder is that Rooney braved the unforgiving world of showbusiness at all. But she’s always been a performer — even before she knew what one was. “When I was three, my aunt took me to see a pantomime,” she recalls. “I was singing and dancing on the seat and this lady leaned over and said, ‘aww, look at her — she’s in her element.’”

Young Rooney didn’t know what the lady meant by “element”, but she did know she loved being the centre of attention, so she rechristened herself. “My gran used to go, ‘You’ll be a great actress,’ and I was like — ‘No, no, I’m Ella Ment!’”

Clad in a chic animal-print dress, eyelids gleaming with emerald eyeshadow, Rooney appears to have little in common with socially awkward, cripplingly insecure Rae. Even so, the character immediately struck a chord. “It’s the first time I’d read a script and thought: this is me.” Not simply because Rae is bigger than a size eight, stresses Rooney, but because — like most teenagers — she yearns to blend in.

“The thing about Rae is that no matter if you’re a boy or a girl, old or young, you can instantly relate to her because you’ve felt like that at some point. But what is normal? Normal is different to every single person.”

It seems TV is finally waking up to that fact; this week also sees the return of the ground-breaking US comedy Girls (Monday, Sky Atlantic), which stars Lena Dunham, another actress who doesn’t fit the mould. Rooney hopes that Rae will be the role model she never had, that “someone going through something similar can see it and think, ‘Maybe I don’t need to hurt myself or force myself to fit in. Maybe tonight I’ll just give myself a wee break because I’m not that bad.’”

My Mad Fat Diary starts tonight at 10:00pm on E4