some of the supporting actors played the same character so differently they might as well have been different characters, sort of a Dr. Whoish situation. I made this joke before in this community but I think it bears repeating, towards the later eps I half expected Bertie to go, "Oh, sorry, I didn't recognize you for a second, old thing, someone keeps changing the actors!"
And the reason Tuppy didn't change is because that actor (I forget his name) happens to be a friend of Hugh and Stephen's.