One great moment or line, in the film....

Is when Costello's pretty much asking Costigan if he's the rat, and Costigan basically says, the first time, fine, I let it pass, the second time, forget it I'm out, 3rd time, if you threaten me I'll put a bullet in you. I think something like that cemented Frank's view on how he could trust Costigan as much as he did, because I guess NONE of his guys would ever say that to him, so, it showed him Costigan had balls.