The acting is absolutely terrible!
David Hemmings was never able to act, bless him. But the rest of the cast are outrageously bad too! Of course it's not helped by Argento's shockingly poor writing, but the whole thing looks like that occasional spoof Spanish soap they used to do on "the Fast Show"
"There's somebody in the house. They're absolutely trying to kill me, you know?"
Dear, oh dear!
But then again, horror fans seem to pride themselves on rejecting traditional cinematic values. It's almost as if they like to turn things the other way up in a kind of "it's so bad, it's good" way. Nothing wrong with that, I guess. As long as they are aware they're doing it. The people who idolise this film must accept, surely, that technically it is utterly dreadful from almost all points of view.