Many an actress will tell you they want to be taken seriously for more than their sexy screen persona. Halle Berry proved that with Monster's Ball a few years back, and now, at the height of her own success, comes another Monster played by the leggy and statuesque Theron.
In person 28-year-old Theron looks ravishing in a slit denim dress. In Monster she gives a remarkable, unrecognisable turn as the gay serial killer Aileen Wuornos. Buffed up and unglamorous, Theron may not have been the obvious choice, but the actress devoured the role. Theron fell short of meeting Wuornos - she was executed just days prior to filming - but Theron read some of the hundreds of letters that Wuornos gave to first-time film maker Patty Jenkins.
Theron, who appeared on the cover of Playboy in May 1999 and was most recently seen as a sexy safebreaker in The Italian Job, also had to put on 30 pounds, wear thick make-up, a set of crooked teeth, and dark contact lenses to mask the brightness of her eyes.
So how much was Theron trying to prove something by playing Wuornos? The defensive actress admits that proving she can step outside a glamorous box was only part of her desire to do this.
"Don't think all of it is that, but I do think it is a little bit like climbing Mt Everest. Why do people do that? I think in life we want to challenge ourselves to see what we are capable of and I think actors tend to do the same thing. I think it's that sportsmanship of 'How far can I push myself?'"
There is also a sense that at last, Hollywood can take one more pretty face a lot more seriously than before. "I didn't quite know... what she was going to look like physically." When she realised just how how different she would look she thought, "Well that would be good, because then people will see me in a different way".
Had Theron met Wuornos she says she wouldn't know how to talk to a woman who had remained so unrepentant. "I think a day before she was executed we talked about it briefly because Patty wrote to her a couple of times and they corresponded to each other, so Patty said 'We should figure out a way for you guys to meet'. Then when she was executed it was the kind of thing where that was done. I couldn't kind of stay in that path and dream about what I could ask her. It was really, how do I now get the information another way."
Asked if it was tough to feel empathy for her, Theron begins by pointing out how hard it is to make a sympathetic serial killer movie, citing others such as Badlands.
"I felt that the other side to her story held a lot of empathy and that it is sometimes hard to look at those things because she had done such horrendous things in her life. I don't think in this movie we try to oversee that or forgot about that but we stayed very true to the fact that she killed innocent people and I think that that's what people have a problem with. But I really believe, otherwise I wouldn't have done this movie, that in that greater truth of her story and watching that, you do get to a place of empathy and that to me was the most important thing."
As well as showing how a homeless prostitute turns serial killer, the film shows how her love affair with Selby (Christina Ricci) gives her hope and how she tries to get her life straight.
Monster may be a small independent film, grisly and depressing at times, for Theron, there is still a certain vindication from the industry that this ex-model from South Africa can do it, backed-up by her recent Golden Globe award and an Oscar nomination for her role.
"I'm getting really honest feedback, which I really like. We do live in a world where people come up and go 'I'm going to tell you straight, I didn't think you had this in you' and that's really nice to know. You really feel that people are honestly giving you feed-back."