Keith telling Will about the Happy Murders in an almost joking fashion comes across as cruel and almost out-of-character for him. Then you recall that Helena mentioned in Boogieman that Keith was actually quite a joker! He's just being open about his previously Hidden Depths.
Sophie is the only one who treats Will's claims of seeing supernatural things as a serious issue (compared to how Keith dismisses these claims). It's not surprising, considering she has the Sandman constantly looking over her.
Kenny Jenkins (who likes to shake hands with good people) draws away whenever Sergio Bowers passed by his cell. This doesn't necessarily imply Sergio is bad, but his surname is very similar to Bowser and if Kenny played any Mario game, he'd easily associate Sergio to the game's Big Bad on the name basis. Keep in mind that we're dealing with an individual, who has severe mental problems.
Any moment when Will meets Ed alone is quite frightening in retrospect. Will was in the presence of a killer, who had already marked Will as his Happy Murder victim. The fact that Will only survived for so long was because Ed didn't know the exact method of it, until Will handed him George Schmid's old notebook.
