The plot has been fleshed out as well. Rather than just be the random focus of assassins, Kutcher will play a successful hit man who gives up his job when he falls for a computer tech (Heigl) and gets hitched. But then he finds out that there's a hit out on him, and he and the Mrs. have to go on the run.
If the trio who have just joined the cast were playing three of the presumed five killers, that would be sweet, but unfortunately their roles are much more typical -- Selleck and O'Hara will play Heigl's overprotective dad and adoring mom, while Mull takes on Holbrook, Kutcher's handler.
Whether this is good or not, at the very least I hope it leads to more Mull on the big screen, and then a return of all those comic greats. It's time Apatow had a showdown with the older comedic royalty.