Harvey

An IMDb Top 250 movie

I never knew about this apparent classic, but it's up there with "Arsenic and Old Lace" as a great example of translating screwball plays into screwball movies. In this case, James Stewart is Elwood P. Dowd, an insane man who is convinced that he is accompanied by a giant rabbit named Harvey.
This idea in itself would get pretty tired pretty soon. But the genius of the story is in the fact that Harvey, real or imagined, creates some kind of impenetrable shield around Dowd, protecting from all who would do him harm. Happily oblivious to the harsh realities around him (he states, in an unusually lucid moment, "I've wrestled with reality for 35 years and I'm happy to state I finally won out over it"), Dowd somehow always gets what he wants.
Anyway, the slapstick moments, as well as situation comedy which actually revolves around the situation (how many 'sitcoms' do these days?), make this one to watch.