You were bombarded with bras at a gig this month. What happened?

A girl at the front said to me: "One time, you burned my bra. Do you remember?" And I totally remembered – I used to be in to burning bras on stage. Once I'd said that, everyone started throwing them. I had bras all around my mic stand. I didn't burn any this time.

In June you put a new track online, Under-Estimate the Girl, very different to your earlier singles – darker, angrier – and it got quite a reaction, both positive and negative. Were you surprised?

I was only thinking it would be heard by the fans that come to my shows. I had no idea it was going to get over 200,000 hits on YouTube or whatever in a week. I can understand why for some people it would seem shocking, really different, because my first album broke into the mainstream in 2007, and people have a memory of me basically being an 18-year-old. That's who they think I am. And I guess, for some people, if you're not in their face all the time, they forget you're a human being and you're probably gonna grow and evolve in six years. Because that's what human beings do.

Paraphrasing that track's opening verse, you sing that everybody plays it safe and nobody messes with the rules. Was that an invitation to think of other artists who haven't evolved, or tried new things?

Yeah, it's my feeling towards a lot of things, [including] the music industry at the minute. When I first started, in 2006, it was an exciting time. Independent, cool, weird artists were being successful, and magazines were writing about them, and people were getting played on radio that were, like, really good. And now everything I hear on the radio is basically a dance track. People have these amazing platforms, and I feel they could do something more interesting. They probably have the talent to do something more interesting, but there's a lot of fear. You have to conform to certain things just to get on the [radio] playlists.

The last time you spoke to the Observer you were launching a scheme, Kate Nash's Rock 'n' Roll For Girls After School Club, that encouraged teenage girls to form bands. What did you learn from your time in British schools?

I've been spending a year and a half meeting teenage girls who just hate themselves. They're really insecure about the way they look, and at the age of 14, dismiss the idea of becoming a musician because of the worry about how the media would treat them. It was such a wake-up call to me. Everybody should be saying: this isn't cool, we have a generation of young people that have no self-esteem. And I thought: we all have a responsibility for this. You do. I do.

What do you mean?

For example, there was this girl I spoke to in Liverpool who was really sweet – tiny, pretty blond girl, beautiful voice, but so shy. She would sometimes skip the [after-school music] class, too scared to come, and I found out that she'd been terribly bullied at her last school. They'd created a Facebook page about her, just full of hate. Everyone slagging her off, really nasty stuff. And it's not like adults can say to those kids: That's not OK, you can't do that. Because we're setting the standard. We watch the grand finale of The X Factor, and we get all these rejected contestants coming on to sing – possibly people with a disability, certainly the elderly – and we all just sit and laugh at them. And then we buy gossip magazines, slagging off celebrities for walking down the street looking human, saying: "Urgh! Look how disgusting and gross they are! They're fat and they're ugly, or they're too thin, look at their wrinkles and their crow's feet." None of the adults who buy into that can turn around to kids and say: "Don't bully each other."

You had your own run-in with Heat after the release of Under-Estimate the Girl, didn't you? It ran a poll, "Who wants the old Kate Nash back?", and you wrote about it on your blog.

It was the perfect reaction [to the release of the song]. It confirmed all of my fears, and it gives me even more drive. To keep going, and keep freaking people out, and fucking with the rules that we're all told we have to live by. Am I gonna give a shit when I'm 85 that Heat magazine ran a poll on me? No. I'm gonna look back on my life and think: I had a really amazing life, I did what I wanted to do, and I was myself.

What do you see yourself doing at 85?

I'll always be playing shows. Even when I'm a crazy granny wearing weird old granny clothes and wandering around with dementia, I'll still be playing. Whether anyone else will turn up is another question.

You once auditioned for the Bristol Old Vic Theatre School, and will finally make your screen debut in a couple of films out this year. Did you miss acting?

I always hoped I'd go back to it but in the right way. I never wanted to be like: "Oh, just because I'm a singer I can be in films now." I got contacted by an acting manager in LA and got to work on a few movies. Small parts, really cool scripts. In one, I play a receptionist in a film about the marketing industry, based on a cult novel called Syrup. The other is about Jeff Buckley, and the tribute gig to his dad, Tim, where Jeff was discovered. I play an English punk.

What's the ultimate goal?

I want to be in a Tarantino movie, more than anything in the world. I'd like to play a badass.

For the last few months you've played with an all-female band. A coincidence, or did you set it up that way?

I was looking for new musicians, and I interviewed men and women. But I was doing all these school projects with girls, and it felt to me like it could really inspire girls to see loads of chicks doing something really well, playing music and enjoying themselves on stage. Now, I'm so happy. We're just such a gang. I've never really felt part of a proper gang before.

Kate Nash's new album is out later this year. She plays Bestival in September