In my book mouth breather is the same thing as slack-jawed and those are both things that describe Josh when confronted with any serious questions about that night Susan disappeared. His jaw drops, he gets this lost, confused, tripping look about him and stutters.
I hear ya. I told my husband that if anything happened to me that he was going down even if he had nothing to do with my death/dissapearance. He is not very well spoken, not college educated. and I can see him bumbling around like Josh Powell if he had a TV camera in his face.