What separates a thought from an action?

So, a little about myself, i think (not diagnosed) and my therapist thinks that i have a form of ocd called pure-o. Basically the obsessive thoughts without compulsions. Due to this mental disorder(?) I have thoughts that scare the crap out of me. Like what if i did this, what if i snapped and killed someone, what if i am a child molester... etc. Anything that would basically scare me, i think of. So, I am just wondering what you think or know about thoughts and actions. Does a thought have to go through a filter in your brain before you act on it? Or is it simply our rationality?