Theory about Johnny (spoilers!)
I expect most of you remember strong scene on roof, when Johnny (unaware he is not alone) speaks his mind about him hitting Lisa. First few times I watched movie, I was convinced that he really did not Lisa, but here are some points that show possiblity him actually hitting Lisa! Some of them exclude another ones.
1) He was drunk. As person who doesn't drink, it is possible he really does not remember doing this. Also, alcohol affect people differet way.
2) He laughed at story about girl in hospital. It shows that he could actually enjoy it, but only could do it when drunk.
3) His monologue on roof might have been prepared. So, maybe he saw Mark being there and still said all of it, only to defend himself more easily when people find out from Lisa.
4) He showed signs of madness. "You are tearing me apart" "Why Lisa why" etc, he showed that sometimes anger is strong with him.
Does this change your point of view when watching movie?