Incest?
This may be a really dumb question but are we supposed to infer that Ray Milland's character has something of a sexual relationship with his daughter? In the early scene where he insists she massage him he's moaning in (sexual?) ecstasy and has a particularly disgusting leer on his face. They then cut to Carrie Snodgress with an understandably distressed expression of her own.
I could interpret it as just his evil, controlling behavior insisting she massage him to exert his power over her and that there's no sexual aspect to it. But the more I watch the movie the more I fill in the blanks and think that maybe the reason he didn't want her to have a boyfriend/husband/chimp/whatever was so he could continue the sexual relationship he'd always had with her. I may be reading too much into it, but I just wondered if anyone else had thought about that interpretation.