Experiment in Terror (1962) 
Long, but lean and tense thriller from Blake Edwards concerning a woman who is being terrorised and blackmailed by a mysterious wheezy stranger who wants her to help him steal $100,000 from the bank where she works. Her teenage sister will pay with her life if she refuses.
Part creepy terroriser, part FBI procedural, Experiment in Terror is a strange mixture of ideas, including (for the time) shocking violence, hints of sexual assault and cross-dressing. However, none of it feels random and it all works well to create a suspenseful little pot-boiler with a great Henry Mancini score.