Veronica Lake, the ultimate special effect, in I Married A Witch, René Clair, 1944.
The insouciant cheekiness of this comedy/fantasy's central conceit allowed the movie to get away with the sort of (near-literal) murder which normlly would have had the Production Code Authority issuing hundreds of kittens. Lake's disinterested and dizzyingly potent erotic presence sold the whole thing as no other presence could. Filmmaker Guy Maddin recounts the stars own sad story in a delightful essay accompanying the new and indispensible Criterion Collection release of the movie. Lake's charm is such that you immediately forget the sad story as soon as she shows up on screen.