The Arrival - S1-E4
Trivia: Michael Cerveris makes his first significant appearance in this episode as The Observer/Bald Man. However if you watch the series from the beginning, he makes fleeting appearances in every episode, sometimes on a tv screen.
Inner Child - S1-E15
Trivia: The Observer September is briefly seen watching the boy as he is driven to his new home. (00:48:05)
Trivia: Olivia walks past an observer on the train while looking for the suspect. (00:42:05)
A New Day in the Old Town - S2-E1
Trivia: The observer quickly crosses the street after the car crash in the beginning of the episode. (00:00:55)
Trivia: In every episode, there is a clue for what the next episode will be about. For example, in episode seven, Samantha Loeb gives Olivia a copy of "A Christmas Carol". In the next episode, Walter is reciting Christmas carols to regain his memory.
Trivia: The story is set mainly in 1985. During a scene in the other universe, the observers are outside of a cinema which is showing Back to the Future - with Eric Stoltz as the star rather than Michael J Fox. Stoltz was originally hired to play Marty, but was fired after filming started and Fox was brought in to play the role of Marty.
Trivia: The Observer appears in the hospital. The agents walk pass him on their way to see the dead man-baby. (00:09:20)
Answer: He helped his father evade capture, impeding the investigation.