Jason Segel (Nick Andopolis) rubbed his eyes for the first few episodes to make his character looked stoned constantly. He had to stop after he had trouble seeing. He can be seen in the background of the opening credits rubbing his eyes.
Busy Phillips (Kim Kelley) doesn't appear in the opening credits, despite the fact that she is in every episode. This was because of contractual issues. Writers of the show justify this by saying her character wouldn't attend school pictures.