I havent watched ER for a few years but it got me to thinking of the early years of ER. According to IMDB.com, these are some of the major characters from the beginning way back in 1994:
Noah Wyle--Dr John Carter
Deezer D--Nurse Malik (pronounced "Maleek") McGrath
Anthony Edwars--Dr Mark Greene
Yvette Freeman--Nurse Haleh Adams
Eriz LaSalle--Dr Peter Benton
Sherry Stringfield--Dr Susan Lewis
Abraham Benrubi--Jerry Markovic (front desk clerk)
Julianna Margulies--Nurse Carol Hathaway
Connie Marie Brazelton--Nurse Connie Oligario
George Clooney--Dr Doug Ross
Troy Evans--Frank Martin (occasional front desk clerk, replacement for Jerry)
William H Macy--Dr David Morgenstern
Christine Harnos--Jennifer Greene (Mark's wife)
Yvonne Zima--Rachel Greene (Mark's daughter)
Beah Richards--Mae Benton (Peter Benton's elderly mother)
There are others but those are the ones I remember most from the list.
And some ER trivia:
-Carol Hathaway (Julianna Margulies) was supposed to be brain-dead from a suicide attempt in the first episode, but the character was revived for the series. For the first episode, Margulies was credited as a guest star.
-If anything had gone wrong during the broadcast of the live episode, such as a technical failure or forgotten dialogue, the producers had additional actors ready to improvise a scene that would have been inserted to cover. This contingency was never used.
-In the live broadcast of 1997, the baseball game that George Clooney's character is watching in the break room was the Cubs-Astros game, also being broadcast live that night on WGN.
-Frank, the desk clerk (Troy Evans), always talks about life on the "force" as a Chicago police officer. In a very early episode he had an appearance as a police officer who was shot and treated by the ER staff.
-Noah Wyle was the last member of the original cast to leave, at the end of the 2004-2005 season. He will appear occasionally in future seasons. Sherry Stringfield is also an original cast member, but she left for 5 seasons, which made Noah Wyle the only cast member to be on the show every year for the series first eleven seasons.
-Including Gloria Reuben and CCH Pounder (who appeared as recurring guest stars during the first season), the original cast combined for 25 nominations in the Leading and Supporting Acting categories at the Emmy Awards from 1995-2000. Julianna Margulies (Best Supporting Actress, 1995) was the only one to ever win, though.
-Doug Ross frequently hung his head low, appearing ashamed or thoughtful or privately amused, depending on the scene. This wasn't just an element of the character: George Clooney had taken to writing his lines on papers, sheets, and other props (especially that complicated medical terminology).
-The set for the pilot episode was a rundown hospital in East Los Angeles, as they couldn't afford to build a proper set of their own. As the rooms were quite small, this necessitated the use of the Steadicam, which has since become the trademark of the show. Real members of the public, usually Punk gangs, would often pull up outside, mistaking the set for the real thing.
-The very first person to appear on-screen in the first episode is Anthony Edwards