A favorite ploy of John Landis. Extras in the zoo scene were told only that David Naughton was going to say a few words to them.
And that blade the dream Nazi zombie is holding up to David's neck was a real knife, held by an actor in a mask that seriously hampered his vision. Most of the fear was probably real there.
Production Posse: The inhabitants of "The Slaughtered Lamb" were described by Griffin Dunne as "half the cast, practically, of Nicholas Nickleby" (the 1980 RSC play adaptation) — notably Lila Kaye (Gladys) played Mrs. Squeers and Mrs. Crummles and John Woodvine (Dr. Hirsch) played Ralph Nickleby.
Throw It In: The little strip of skin flapping about on Griffin Dunne's face when the undead Jack visits David in the hospital was kept intact in the final cut when it was found to have made Dunne's appearance far more convincing.