I know this is Repetitive but For me One Willow Wish didn't created the film's content and theme but instead it was already there, all it did was sparked and amplified it. In My Opinion Nikki have a crush on Bear, possibly huge, and already wanted to be with him. I think one of the reasons why she wanted to stay with him longer, for example inside the car, was to hear Bear would asked her for a date. Nikki possibly have Obsessive Tendencies, based on Ian's statements that she's crazy. I think Bear just misread the situation, for me if he only just admitted about his feelings then ask to spend time with her i think she could still said yes, engages in relationship and also her being obsessive with him with lesser Intensity.

The Writer possibly initially wanted Sarah to be the Villain but it would leave a huge Content Gap in the Script. I think they realized there wouldn't be a Parallel Character and would also made the film far more darker, disturbing and also expensive to made.

Also my last generic Theory is that Sarah Obsessive will be a far more dangerous and brutal Force than Nikki because of her Calm and Calculated Demeanor, based on the Actress who played her she already have a huge crush on him considering they're been friends longer, so she could most likely have caused more destruction. There's also the possibility of if wasn't Nikki then it could be Sarah later on.

What Best Non-Tragic Theory in Obsession that make Sense in your Opinion?