I have only recently begun to watch Fringe. I just finished season one and am two episodes into season two. I have noticed an Observer in most episodes of the series. Sometimes the Observer is plainly obvious even interacting with principal characters. Other times the Observer is seen only briefly or is hidden in the background.
Is an Observer (not sure if there is only one or many at this point) in every episode? If not every episode is there a list of the episodes where the Observer does not appear? Does the Observer only show up for particular types of events?