I appreciate your points. I didn't find Julián was a "seemingly normal, well-adjusted person" at all, though.
He is a single, 40-something man who regularly clips photos of beautiful, very thin women from foreign magazines. He becomes obsessed with his friend's wife. At one point he urges Ana to use the rowing machine at his place to make herself thinner, and pushes her to do 100 reps. Then he tells her she has to shower, in his apartment, after the event. Then he pricks her with a needle, and starts kissing her where he hurt her. This is just creepy stuff.
It becomes clear, after watching his behavior, that something darker exists beneath the surface, and will eventually erupt. It is very Chabrol-esque.