This article is from the ER FAQ, by Rose Cooper cooper@acm.cse.msu.edu with numerous contributions by others.
2.11 Main cast:
Dr. Mark Greene, played by Anthony Edwards
ER Attending physician
Dr. Peter Benton, played by Eriq LaSalle
Surgical Resident, sixth-year (season 5)
Dr. Doug Ross, played by George Clooney
Pediatric ER Attending physician; engaged to Carol
Dr. John Carter, played by Noah Wyle
Resident, third-year
(second-year ER Resident; season 5)
Nurse Carol Hathaway, played by Julianna Margulies
Nurse Manager; also runs free-clinic; engaged to Doug
Dr. Kerry Weaver, played by Laura Innes
ER Attending physician
Jeanie Boulet, played by Gloria Reuben
PA (physician assistant) working in the ER
Dr. Elizabeth Corday, played by Alex Kingston
Surgical Resident, fifth-year (season 4);
Surgical Intern, first-year (season 5)
[It's complicated; it involves Math.]
Lucy Knight, played by Kellie Martin
third-year medical student
[...and, "young female doctor void"-filler] (season 5)
 
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