Maggie Grace, was initially cast to play Dr. Rorish, the ER Residency Director. Grace exited the show, before it had begun.
Marcia Gay Harden, was originally cast to play Christa, one of the first-year Residents. However, when 'Maggie Grace' left, Harden was offered and accepted the role of the ER Residency Director.
This show tries to convey to us, as viewers, an unprecedented look into what a large hospital's trauma area, can be like; both as a visitor, and as a staff member. The Documentary that preceded this show, is based on the Los Angeles County Hospital's ER. Its trauma area, has been described as, both the birthplace of Emergency Medicine, where 'more people have died and more people have been saved, than in any other square footage in the United States'. [codeblackmovie]
The Pilot episode was shot in the REAL LAC+USC trauma center in Los Angeles. All following episodes were on set but the looks were inspired by the LAC+USC trauma center.
Bonnie Somerville, was the last to be cast. Due to the departure of Maggie Grace, Marcia Gay Harden, took over the role of Dr. Leanne Rorish. This left an opening for Sommerville to be cast as Christa.