One film that, though British, is a must is VILLAGE OF THE DAMNED (1960) based
on John Wyndham's THE MIDWYCH CUCKOOS.  Also, you might want to consult
several episodes of the original STAR TREK (the one with Melvin Belli as
The Gorgon) and THE TWILIGHT ZONE (especially the episode that was remade
for the John Landis film).