Sharon Rooney had always played her acting career for laughs. But when a good friend died she couldn't hold back the pain. That's when her life changed

We'll have to wait to find out because the fact is, Rooney has made it as an actor. The lead role in the Channel 4 Britpop-era drama My Mad Fat Diary - in which she plays Rae, a Lincolnshire teenager who can add a spell in a psychiatric hospital to all the usual teenage problems - means she's now recognised in the street, has more than 100,000 Twitter followers and has been named a Breakthrough Brit by Bafta, with Olivia Coleman (her hero) as her mentor. Oh and she's got a role in new BBC Scotland sitcom pilot, Miller's Mountain - written by Donald McLeary and set in and around a mountain rescue team, led by Jimmy Chisholm. Rooney gets to play Scottish in that one. Despite her way with a Lincolnshire accent, Rooney is a Glaswegian. "It was nice to be surrounded by Scottish accents and I could say, 'whit?' and no-one would get instantly offended."

But there is something palliative about her breakthrough role actually. Because playing Rae - who is based on Rae Earl, the woman who wrote the real-life diary of the title - has clearly connected with people. So yes, some recognise her in the street. And some do more. Some feel the need to tell her their stories of pain, failure, mental fragility and feelings of inadequacy. "And it's not just young people," she says as we sit on the top floor of the BBC building at Pacific Quay. "It's people my age, people in their 30s, 40s, 50s, 60s."

And that can be tough, she says, because in the end she is just an actor. "I don't have the tools to help people. When you've got someone face to face that you don't know who's telling you their life story that's sometimes horrendous. You have to go, 'I'm so sorry. I've got no answers.' That's hard."

Certainly playing someone with Rae's issues has opened Rooney's eyes. "I think now I'm more aware. Mental illness affects one in three." She casts her eyes round the busy BBC cafe. "Look at the people in this room. That's quite a lot of people."

Even so, it's good to talk. What's great about Rae, Rooney says, is she has so much going on. She's very relatable. "I get a lot of mums who come up and say, 'I'm so grateful because now I get it. I know how to speak to my kid, how to approach them. It's not just teenage behaviour. It's not attention-seeking.' That's quite cool."

Is Rae relatable to Rooney, though? Because fragile is the last thing Rooney comes across as. She says she's shy and something of a worrier but in conversation she's funny, self-deprecating and self-confident. If Rae struggles with her physicality and sense of self, Rooney seems perfectly at home in her own skin.

We live in a culture that too often problematises appearance. Advertisers and parts of the media tell us we're too fat, too thin, too black, too white, too old. A drama like My Mad Fat Diary is a reminder that such stereotypes need to be challenged. Rooney offers another approach. You can simply ignore that stuff.

Does she, because of her success as Rae, see herself as a poster girl for normal young women? "The only thing I'm a poster girl for is being yourself. I wouldn't sit here and say everybody should be my size. Or everyone should be a size eight. I don't care what size you are as long as you're happy and healthy. I like people for their attitude, not for the way they look."

It would have been good growing up to see someone who wasn't "stereotypically pretty or the stereotypical size", she says, but "I never sat and thought, 'Oh I wish there was someone with bigger hips.' It never crossed my mind. It never really bothered me.

"I've never been smaller or bigger than I am now. It never held me back, but I think that's because I never let it. I never wanted to be like anyone else. I think I'm just lucky that I've grown up in a family that's been filled with love and I've never felt not good enough. I've never felt not pretty enough."

Rightly so. Rooney is clearly close to her parents. She still lives with them. She's considered moving down south but home's free, she points out. She has a close group of friends in Glasgow and the alternative would probably mean living in a hotel for months at a time. "I like to be at home where I can get Irn Bru whenever I want."

Plus, it means she can spend Saturday nights playing electronic Monopoly with her parents. Last Saturday she lost, mostly because she sold her dad Park Lane. "I'm quite dull. I'm 25, I play Monopoly on a Saturday night with my mum and dad - and lose."

Her parents are the only thing she's cagey about. Dad's a really good businessman is all she'll say. Obviously: he bought Park Lane off you. "I'm scared you're going to write he's in the Mafia. He's not in the Mafia, or I would be dressed in designers."

She even watches My Mad Fat Diary with him. That must be fun. I used to be embarrassed watching sex scenes with my parents and at least I wasn't in them. "I don't see any of me in Rae at all. It's not like watching me back. People go, 'How can you watch it? Is it not embarrassing?' I don't see it as me, thank goodness, because I wouldn't want to watch myself having some fun times. That would be weird."

The possibility of a nursing career aside, Rooney has always wanted to be a performer. She tells me a story about being taken by her aunt to see a panto aged three, standing up on the seat and dancing along. "This old lady leaned over to my aunt and said, 'Look at her, she's in her element' and I just heard 'element' and that became my stage name. Ella Mint. Even now my mum will send me cards 'To Ella'."

As a kid, Rooney put on shows - Michael Jackson impersonations, mostly - for her grandparents. She studied drama in Glasgow and then Hull, which was so theory-based she was bored most of the time. It possibly didn't help that she's dyslexic, though she wasn't diagnosed until she was in Hull. She came home to work in shops (she was working in Home Bargains when she heard she'd got the audition for My Mad Fat Diary), and for Theatre In Education, putting on shows for school kids.

She even had a go at stand-up because that's what her best friend Anna does. "It was probably the skintest I've ever been, but the happiest. Some gigs were hellish, others were great. I decided to sing. You know those moments where you have a complete lapse of judgement? Out I'd come in a wee skirt and I swear you could hear people go 'aww' and women are saying to their partners 'shoosh, let the wee lassie sing'. I used to sing comedy songs." Emeli Sande was once her support act.

She makes life sound like one fun thing after another. And it is. Mostly. When she started auditioning for My Mad Fat Diary she thought: "I'll just be funny, I'll walk in here and I'll be funny because my whole life I've always been the funny one. I've always played funny parts. I've never done anything serious so my thing was, 'I'll just make them laugh' and every single audition, that's what I'd do. I'd go in and be funny.

"And then on my last audition, before I went in I got a phone call saying one of my friends had passed away."

Was it a sudden death? "Yeah, she hadn't been poorly, it was just ... Life works in ways sometimes you don't understand.

"I'm better now at showing my emotions but two years ago if you'd asked me to tell you a sad story about my life I would have gone, 'I don't have any. My life's good."

The phone call about her friend changed that. "I had spoken to her on the Monday and the last thing she said to me was, 'Don't come home without this job, go in and fight for it.' Two days later, sitting in the audition room, I got this phone call and in that moment it was fight or flight. It was either go, or go in and don't worry about letting your guard down.

"They were so good about it. 'We'll get you home', but I was, 'No, no, no, I need to focus, because she would have been so mad if she had found out I'd walked away from the chance of a lifetime. There's never going to be another part for me like Rae. In that moment I had to break the barriers down and show them some vulnerability and I think that's what got me the job."

Is it too much to say Diary has changed her life? "No, I think it definitely did." There was a time when the only roles she got offered were the plus-sized bully or the vulnerable girl. "It was always, 'You are the bully or the victim."

That's changing now. She's still to hear if there will be a third series of Diary but she's got a play this autumn and as Jules in Miller's Mountain - a traditional sitcom performed in front of a live audience - she's the barmaid and gets many of the best lines. "It's probably one of the funniest jobs I've ever done," she says. And she's always wanted to pull a pint. "I've no idea how to pull a pint. I've never even drunk a pint. I don't drink."

Maybe Rooney's part of a wider change. After all, she's not unique these days. Turn on the TV or go to the movies and you can see women like Ruth Jones, Rebel Wilson and Lena Dunham. The airbrushed stereotype is being challenged and the definition of beauty is being redefined. "Ruth Jones is stunning. Lena Dunham is stunning ... I think she's stunning. You might not ..." I nod. "I'm glad you agree."

Has success made Rooney more confident though? She thinks not. If anything she was more sure of herself when she was 15. "I was dancing every week, had a great group of friends, I was pretty happy. I had a boyfriend but it was one of those things where we went out with each other for three years but nothing ever happened except a couple of kisses. I know that some people don't have a great time at school. But I always wanted to act and no-one ever told me it was silly."

Sharon Rooney has grown up to be an actor. No-one is saying that's silly. I think that qualifies as a happy ending. Patients requiring catheters may need to wait a while longer.

Miller's Mountain is on BBC1 at 10.35pm on Tuesday