1. Creating empathy for the characters.
I didn't know who to care for - the Mom, played by Tilda Swinton, who at times seemed like Mommy Dearest, yet at others (most, actually) was pretty normal. Do I feel bad for her that she has this nasty kid? Or, as the unreliable narrator - do I question everything she sees or says? Or, do I feel empathy for little Kevin, brilliantly acted by Ezra Miller, who had a mother who disliked him because his birth ruined her carefree life? But then I see Kevin being so manipulative, even at a young age, that I start to think he was born a sociopath.
Anyway, bottom line, I didn't care about what happened to either one. Meaning? I could have walked out halfway through the movie. (Never mind that I was home watching it on Netflix. You get what I mean.)
The movie failed to commit to a story concept: psychological thriller ala The Hand That Rocks The Cradle or Fatal Attraction or horror ala Exorcist. So, it wavered, never really going all out on either one to make it a great movie. It certainly had the potential to be the former but then it was as if the directors and producers gave in to the demands of Hollywood and 'dummified' the movie for greater appeal.
Lesson for us authors? Pick your poison. Choose a side. Commit. Fill in your chosen cliche here. Even if you choose wrong, at least the reader knows exactly what you were going after - even if, in the end, you failed at executing.