The character of Doug was only supposed to be on the pilot, but the producers liked Kevin Nealon's performance and the comedic aspect of Doug so much that kept him on the show.
Jenji Kohan explains on the DVD commentary that the decision of literally burning down Agrestic and move on to another story-line was due to the fact that the writers were getting bored of writing suburbia jokes. Mary-Louise Parker was particularly pleased with the idea of killing the premise of the show, as she loves risks when it comes to her acting.
For most of season 7, Jennifer Jason Leigh was doing a play in New York and wasn't available, so the writers came up with the idea of having her character appeared in video chat sessions. She filmed those with the director at her house of at breaks from the play.
During seasons 2 through 5, several episodes feature a DEA agent named Roy Till with the rank of Captain. The Drug Enforcement Administration, a federal agency within the Department of Justice, has no such rank.
Her plotline is left unresolved after Season 5, but series creator Jenji Kohan has stated that Celia went back to Agrestic, and fulfilled her desire to be like Nancy by becoming a drug dealer.