This movie charts the friendship of Holly (Williams) and Marina (Anna Friel) over the years. Holly is the mousy nerd to Marina's party girl, and inevitably tensions arise, particularly when it comes to sleeping with each other's boyfriends, brothers, etc.
Williams demonstrates a decent grasp of the British accent, but there's not much to recommend here. The pair are so antagonistic, it's hard to believe there could be a shred of genuine friendship between them.
No, she obviously had to go brunette to play the dowdy, bookish one.