The movie Mr. Brooks (2007) ends with the main character, Brooks, having a nightmare of his daughter killing him, confirming his fear of her becoming more and more like him. After which Mr. Brooks wakes up, realizing this was a nightmare, being comforted by his wife and reciting the serenity prayer.
How do we know if Mr. Brooks' nightmare was only that his daughter killed him, or he was dreaming this whole time? Is this an open ended movie, where we simply don't know? Or am I missing something and it's actually close ended, where the dream was simply the last part and not the whole action.
Reasons to consider: We know Mr. Brooks has not killed anyone in two years, and it looks like the serenity prayer is his sort of "medicine", to take in hard times of temptation. So maybe he recited the serenity prayer to stop himself, had a bad nightmare about killing again, woke up, and recited the serenity prayer again.