I hate being this guy... But when I was a kid I saw a movie on TV and to this day the image of it still hunts my brain.
There is a kid, standing at the top of some stairs outside what might be an entrance to a basement, he is talking to his grandfather (Older man, a bit pudgy, Recognizable from other movies of the time frame, but not famous persay!) As the camera pans down the stairs you see that the grandfather is dead, and behind the wall, out of the kids line of sight the Alien has it's tentacles up his spine and is speaking for him!
That's it... That's all I remember, and to be honest it might not have been an Alien, it could have been a monster of some kind... I have been searching for a while, hell when I was in High School I even got a job at Suncoast Movies to see if that would help me find this thing!