Well, you know when writers do this (just making up an example).
John made an excuse and left the room. So that's it, thought Sarah. He's trying to get out of washing up after himself.
It's obvious, it's crude, it's not good practice. Better (but still not great) would be:
John made an excuse and left the room. Sarah felt that his departure was simply motivated by a reluctance to wash up after himself.
EDIT: I say thought bubble because the thought trains which go into italics tend to be specific and contained, much like a comic's thought bubble.