At least in my own experience, role plays tend to go sour for one of four reasons:
1) One of the participants takes the canon of your setting and characters way too seriously.
2) The neighbors refuse to cooperate.
3) The giggles.
4) The role play itself is too close to what one of the participants actually does for a living. Like this:
I’ve been guilty of “pulling a Schrader” once or twice myself, I will admit. And I know what the other side feels like, too.