There was a lot of behind the scenes stuff with regards to the character of Doyle, not the least of which was that the actor who played him, Glenn Quinn, was an addict. He died of an overdose a couple of years ago. Public spin (because ME really are a bunch of mensches when it comes to keeping personal things personal for their actors) is that they had always planned to kill Doyle off. The truth is he was fired for being incapable of doing his job. I'm sure this had a huge impact on his role, the arc, and how he played off the other characters/actors in general. Him and DB were really, really tight. DB won't talk about him much, even when asked, and sometimes he still tears up.