This is one thing I'm struggling with right now. Perhaps its just an author by author basis, but perhaps there is a right and wrong way to do it?
I notice in some novels, the author will say something like,
"John closed the door,but then stopped in his tracks, I can't believe I forgot it,"
This italic, as if we're hearing inside the characters head is very common. I see it in a lot of the books I read, but then I see it expressed differently in others.
"John closed the door, stopped and groaned. How on earth could he of forgotten the book?"
I see this done a lot too, where the narrator is guiding the thoughts rather then letting the character just say the thoughts. I feel the latter is bordering on telling, rather then showing, but I see it in a lot of great fiction and can be used very effectively as it adds to the voice of the work. So I'm kinda trumped as to which way to go. Right now, I seem to just be jumping between them.