It's interesting that you say that, because fanon generally has it that Snape and Filch are friends. Mostly based on the fact that when Snape gets his leg bit by Fluffy in PS/SS, he doesn't go to the infirmary, but to Filch to have it bandaged up. The assumption tends to be that he trusts Filch not to tell anyone.

But they're not obviously buddies. The other time we see them talking when (they think) they're alone is in GoF, and they argue. Interestingly, though, they're really after the same goal -- they want to punish someone who's made trouble in the school, they just disagree over who it should be.

This fandom is so thick with analysis and long-held assumptions that it's often enlightening to get the perspective of someone who just read the books as books. *g*