The Tingler (1959), produced and directed by William Castle.
For all its cheeziness, this is in many respects quite clever and is a favorite of mine. I first saw it as a child late at night on a old boxy TV in blacked-out room. The deaf-mute woman's freakout scene really was the stuff of nightmares back then.
The cheeziness: you can see threads pulling the creature along.
The cleverness:
The concept is a good one: everyone has a microscopic parasite at the base of the spine which grows enormously during the experience of terror...
...and can only be vanquished by screaming.
What if a person couldn't scream? Say a deaf-mute woman with a blood phobia that could be used to frighten her to death? In that case a full-sized Tingler could be extracted and studied...
...if it weren't so malicious and curiously indestructable.
Is medical examiner and fear-specialist Vincent Price bad enough to do something like that? Good plot twists here.
He makes history with the first film use of LSD, in this case to cause "waking nightmares".
You have to imagine you are in the theater for the climax. The Tingler is loose in the theater in the film, so of course everyone watching the film is checking under their seats...
...while hired shills scream and faint and are carried out on stretchers...
...when the screen and theater go black and Vincent Price announces "The Tingler is loose in the theater! Scream! Scream for your lives!"...
...and Castle deploys his "Percepto!" gimmick, which are electric buzzers installed in certain seats...
...and when the creature attacks the projection booth the film breaks and we see its magnified outline crawling across the screen.
That must have been fun.
Notes:
We last saw the deaf-mute woman in another non-speaking role: Miss Lonelyhearts in Rear Window (1954).
Some of the music cues are swiped directly from Herrmann and Vertigo (1958).
For some reason we see quite a bit of the silent classic playing at the theater: Tol'able David (1921).
The title of the LSD book is printed on the back cover: Fright Effects Induced by Injection of Lysergic Acid LSD25. I can't explain it; this is the bizarro world of William Castle.
Available on DVD.