Trauma director Marc Evans answered that question at the Trauma Press Conference on August 24, 2004 in London.

[MARC EVANS] WHY WAS MENA SUVARI CAST AS CHARLOTTE?
Just her quality, her otherness. We wanted this character to be a potential angel in Ben’s life and she’s a well-meaning neighbour, beautiful and different. Mena has all those things in spades, but she also has a dark streak and is interested in this kind of material. She’s great to work with. And as for the American thing, the part wasn’t written as an American, but when she said she was interested I saw no reason to make her English and in some ways it was an advantage, as she has to talk about crystals and alternative medicine, and in a funny sort of way that seems to come more easily from an American character than perhaps an English one. It seemed to sit very easily with her. (Found at Firth.com)