movie did the guy put his fingernail between teeth

a nail-biting experimental film (literally)

A man has a nightmare in which he compulsively, violently chews his nails and cannot seem to stop.

Without a doubt, Willem Dafoe really likes his gappy teeth.

1981, 16mm, b&w, silent, 2 mins. Shots of Halpern cutting her fingernails, biting them, and dunking them in water.

There‘s a finger. She bites it.