She really nailed the "nice girl" routine so even with all the little hints spread throughout the movie (Doug's phone number was on her desk), it's still somewhat of a shocker that she was the mastermind.

I agree with SentinelPrime. I thought she was excellent at both sides of the character's personality; her sweet, innocent pushover persona we see at the start and feel sympathy for, and then her deliciously dark side that most of us love just as much!