On the small screen, she was the quintessential all-American cheerleader, both in Buffy the Vampire Slayer and Veronica Mars. It was a role for she'd had plenty of real-life experience as a San Diego Chargers cheerleader in the early 90s.

But despite starting her TV career in 1995, with a guest spot on Baywatch, and seeing plenty of success in that medium, the 39-year-old actress has yet to properly crack features. That's all set to change this year. First up is a British horror film, Psychosis, which is what has brought her to London for an exclusive chat with Cinematical today.

And later next month she'll play Jason Statham's character's girlfriend in Sylvester Stallone's The Expendables. Despite a 15-year career, it's her first major Hollywood blockbuster. She sits down to discuss the film and update us on those rumours about big screen translations for Buffy and Veronica Mars...

Psychosis is a small-scale British horror film, how did you find it?

I think because of Buffy and Angel and Joss [Whedon], and his schooling being from England, I think there's probably a natural gravitation towards working here. You're a neat group of people and you gravitate towards quality – I think you knew it when you saw it with him. Because of those shows, England's been good to me. So they called and offered the role of Susan to me. I read the script and of course everything starts from the page. I liked it and I think what sealed the deal was the conversation I had with [director] Reg Traviss about the questions I had with the story. Why does she have dinner with him? Let's talk about that sex-scene! [laughs] How is that going to be filmed? It was just that he was very informed. I think whenever you can get with the originator or the creator and they happen to direct, they make the best directors. Their understanding of the material is greater and deeper than what you'd do with it. I love working with directors who are able to inform you with more and give you a better performance because of that.

You're known for your TV work and you've done a lot of great work on TV. You've only just started getting into features. Why is that?

Had I been smarter when I was younger and I had the success of Buffy and Angel, I'd have parlayed my breaks into a film career, like Kristen Bell did on Veronica Mars. That was a really well written role and well executed by her. She'd make a movie in her break. And the show didn't go for seven years, so it didn't stifle her. It was the perfect way to launch her, get her known, get her seen, and ease into it and have the success she's had. So I think if I'd been smarter when I was younger, and more ambitious, I'd probably have been in a better position today, or today would have happened sooner. But it happened exactly as it was supposed to happen with me, because nine months of that for so many years – I wanted more balance than that. I took my holidays and I ran with them. I travelled, I stayed home, I had a baby, you know, I did everything the way you're not supposed to do it if you're an ambitious young woman!

Charisma Carpenter in Veronica Mars

You're off a great start with Psychosis and The Expendables.

I know! My agent says [The Expendables is] amazing! No, he says it's "frikken awesome!" But I wanted to get in on the action! If there is - and I hope there is - going to be an Expendables 2, I'm going to hit Sly up really hard and say, "I really want to go and I really want to kick some ass!"

How was the experience of working with Sylvester Stallone as a director?

Oh, it's insane, you know, I don't know how he does it – how he wears so many hats. I could not function in that scenario, but he, fortunately, doesn't need a lot of sleep, is high-functioning and is able to wear those hats and be really, really successful. You know, he's been around a while, so I think he probably knows a thing or two about how to do that stuff. It was really, really surreal to be with somebody that you've watched growing up. And my dad loves Sylvester Stallone, so I grew up watching the Rockys, loved him, loved Copland. I was a fan. To be on a movie set and be directed and in a trailer discussing the character and discussing the part and having a say on what I want to wear – giving me so much personal time – it was shocking, you know, and not what you hear. But he's very hands on.

Who are your main co-stars in the movie? (Beware spoilers!)

It's Jason. I play his girlfriend. We're having a bit of a problem, because he's an Expendable – a mercenary – that's a job that is, "I'm going away, baby, and I don't know when I'll be home. Can't tell you where I'm going or when I'll be back," and I'm a nice girl and I want to have a life and I deserve a life and to be with somebody. Therein lies the rub, where he comes home unexpectedly to see me and... it's not a good time! Surprise!

Buffy and Veronica Mars are both TV shows that have been mooted for possible movie spin-offs. Would you be keen to be involved?

To be honest with you, I've never heard that and I don't know if I could really see Kristen doing that. But to work with Rob Thomas or Joss Whedon again, either way, would be fantastic.

Joss is doing The Avengers – is there maybe a role in there for you?

[laughs] I don't think so, he hasn't called! I don't want to pester him – it was enough with Wonder Woman!

The Expendables hits theaters on August 13th. Psychosis is available now on DVD in the UK.