Charlize Theron goes so deep into her role as Aileen Wuornos in the 2003 film 'Monster' that she's almost unrecognizable. In real life at the time she was dating Stuart Townsend, but she had no difficulty convincing the world she was a lesbian in the movie. Her dedication to the part is absolute, and it's no surprise that it won her an Oscar.
In 'Monster' Theron's character is a mentally-broken prostitute who snaps and goes on a murderous rampage. Stripped of her makeup, bathed in sweat and wearing a permanently unhinged expression, Theron completely transforms into her part in a way that surprised and delighted critics. The film is a biopic, and thanks to Theron's acting abilities, the resemblance between her and the real-life Wuornos is striking. It's a tragic tale with a tragic ending, but it's an incredible movie.