Blindside (1987)

First, he was paid to watch her. Now, he'd pay any price to have her.

Harvey Keitel plays Penfield Gruber, a once great scientist, reduced to managing a sleazy hotel. Gruber monitors the daily comings and goings of his tenants, mainly for his own interest, until underworld figures ask him to spy on a suspected double-crosser. While watching the man, Gruber overhears a murder plot.