Michael Ansara
Finally saw the movie for the first time this weekend (Sept. 2015). It's great when TCM runs the 60's/70's oddities in the middle of the night.
Michael Ansara, one of those character actors (I remember him as 'Kane' on "Buck Rogers" reruns) with a respectable 50-odd years in Hollywood to his name, was credited as a "It's Alive" co-star with fifth or sixth solo billing in the opening . . .
. . . and then his 'role' consisted of three lines in one brief scene at the 35-minute mark.
Weird.