backseat dm
The "backseat dm" or "backseat dungeon master" brings a thoroughly marked and noted rulebook, and is quick to jump in whenever a rule is broken or ignored.
The nerd/D&D version of backseat driving
Player 1: That's not really how it's done, but I'll allow it.
Player 2: Wait a minute! That's against rule 534 on page *flips open rulebook*
P1: I'M the DM here. Don't backseat dm me.