Filling interior door trim gap
Find some cheap 1/2"x (something like 1" -- could be 3/4; could probably be as big as 1-1/4). Nail that to the jamb first. (1/4" reveal is typical.) Then put door casing on top of that. Could be dead flush, or could have a slight reveal of its own. 

Pro tip: too small a reveal (like, 1/8") is hard to do, because subtle differences in width can be seen easily. A bit bigger, and your 32nds disappear.