When an investigator enters a room that is on fire, he must do a Horror check. What about when an investigator is in a room that catches on fire, must they roll a Horror check then, too?

(I accidentally posted this in the PoD forums as well. Please forgive the double post.)

No, only when you enter a room on fire.

That's pretty much what I've done. Maybe when the fire starts, it isn't that big, and by the end of the turn it's big enough to freak out anyone entering the room.