Trailer Single White Female and possible ideas?

This trailer shows a particular idea we got from our teacher in that we could have our film based round a psychotic character who 'would kill to be her', that is her room mate. This would be fairly easy to film and the trailer would conventionally follow the rising tension as it progresses. This would mean we would need to cast 2 young girls to fit the style of this story. This would mean finding a girl the same age as me as we could use myself as an actress in this. film.