The Spiral Staircase (1945, dir. Robert Siodmak, adapted from Ethel Lina White’s The Spiral Staircase) (via)
"This sort of lunatic is usually normal in between his fits of mania. He might be living in this house with you, and you’d accept him." 
Helen shuddered. 
"Are you trying to tell me someone in this house committed all the murders?"
"That is the paralysing part of it," insisted the doctor. "Just imagine the horror of seeing a friendly, familiar face—like my own—suddenly change into an unfamiliar mask—with murder glaring out of its eyes?"
-Ethel Lina White, The Spiral Staircase (1933) 

The Spiral Staircase (1945, dir. Robert Siodmak, adapted from Ethel Lina White’s The Spiral Staircase) (via)

"This sort of lunatic is usually normal in between his fits of mania. He might be living in this house with you, and you’d accept him." 

Helen shuddered. 

"Are you trying to tell me someone in this house committed all the murders?"

"That is the paralysing part of it," insisted the doctor. "Just imagine the horror of seeing a friendly, familiar face—like my own—suddenly change into an unfamiliar mask—with murder glaring out of its eyes?"

-Ethel Lina White, The Spiral Staircase (1933)