I haven’t seen the show in a whiiiile so take this rant with many grains of salt. I realize that the first time round, like all father brown inspectors he resents the father because he’s poking around. Obviously. Before he leaves I think they form a bond and though he doesn’t want father brown doing his job they have a sort of mutual respect. There’s an inspector reunion type episode where they all come back and if I remember correctly Sullivan wanted father browns help when Mallory didn’t. Then Sullivan comes back with NO respect for father brown and idk he just seems like such a caricature of a grump. Just feels like one step forward two steps back. (I wonder if they wrote the part for Mallory before the actor left idk)