I was looking for a book I read either late 60's or 70's. It wa a sci/fi and psycho thriller all in one. The most interesting detail I remember is that the entire book takes place on a staircase. They think they are the only one there but find out at the end that there were several people there all participating in this psycho experiment. I don't remember the title or author but if these details remind you of a book from long ago I would appreciate it.