Don't Bother to Knock (1952)

Airline pilot Jed stays at the New York hotel where girlfriend Lyn is a singer. He sees Nell in a window opposite his and they get chummy. When the girl she's baby-sitting, Bunny, enters Nell goes crazy and sends her to her room. She fantasizes that Jed is her long lost fiance. Jed comes to realize that Nell is more than a little whacko.