I always thought Frank the rabbit was a Harvey nod. Richard Kelly says he hadn’t even seen the movie.

Both films put a roughly six-foot rabbit next to a man other people think is unwell. Jimmy Stewart’s Elwood P. Dowd in Harvey (1950). Jake Gyllenhaal’s Donnie. Same setup. Opposite rabbits.

Harvey, from Mary Chase’s 1944 play, is a white pooka: 6 foot 3½ inches, and invisible. The audience almost never sees a rabbit body. Frank is a Halloween costume. James Duval wears it. April Ferry built the skull-grin suit from Kelly’s sketches. Kelly wanted it disturbing and animalistic, not an Easter bunny. He has said the rabbit pick came from Watership Down: an innocent animal made monstrous.

So the appearance link is an inversion. Unseen white pooka vs seen grotesque mask. Same height class. Not a quote of Harvey. Kelly denied that in 2002 and again in 2017. Wikipedia still notes the popular belief, and the Aero Theatre in Santa Monica even ran the two films as a double bill in 2002.

No shared casting either. Harvey is not a man in a suit. Frank is.