Background characters should not be mistaken for the stock character type known as "redshirts", for which actors are usually hired specifically. For more detailed information on this distinction, see the "Further clarification" section below.
The middle section background was originally cast as 7 survivors in addition to the main cast, but Damon Lindelof and J.J. Abrams eventually convinced ABC to allow them to expand the background cast to 33, to have a total of 48 survivors . Therefore the recurring middle section background pool was later expanded to encompass several dozen. These background characters recur on the island across episodes, and even appear in flashbacks to the Sydney Airport before boarding Flight 815. Due to availability of the actors (such as moving away from Hawaii, where Lost is filmed), there has been some turnover in the middle section background cast.