Julie Gray, a screenwriter who blogs about screenwriting, makes an interesting point in her post entitled 'The Angelina Jolie Effect'. She touches on the fact that in Jolie's latest film, Changeling, she is supposed to play an ordinary woman caught up in the extraordinary drama of having her kidnapped son replaced by another boy. The problem is (as Gray points out) that Jolie looks nothing like an ordinary woman. She is so otherworldly looking that it's impossible to buy her in this role--which apparently derails the whole film.
I haven't seen the movie, but it really doesn't matter. Gray makes a very valid point that doesn't have to be about Hollywood and her films to be worthwhile. When casting for whatever role, care must be taken to make sure that the look someone has makes sense for the part. You don't cast a homely person as a supermodel, a short person as a basketball player, or a thin person as someone who is supposed to be obese. There are exceptions to every rule (casting against type is often very effective in comedy), but common sense should prevail here.
That's why you hold auditions. If you can't find what your looking for in your talent pool, extend the sphere of your production by holding open auditions to attract people you have never met before. Don't settle for someone you know is wrong for a part just because you think no one else is available. Your project is only as good as it's weakest link, and if a viewer reacts negatively to talent before they even open their mouth, you've permanently taken them out of the experience.