SPOILERS AHEAD
I loved this movie. I guessed the ending probably in the first 20 minutes because of the parents' erratic, strung out behavior.
It's kind of nonsensical when people say that the acting in this movie was bad because the couple "acted like they were on drugs" and "didn't seem to show genuine concern and worry for the child". It seems like they completely missed the point of the movie. It actually would've made less sense if the couple had maintained a typical "worried parent" response to their child being missing. They're incredibly mentally unstable; their behavior is not going to be that of somebody whose child has just gone missing, because they know deep down that she has been dead for years. They're living in a delusion, and the two main actors did a wonderful job of portraying this. For anybody who couldn't guess the ending, their turbulent behavior should've made even more sense after they saw the big reveal.