24 Hours (ER)

• Anthony Edwards as Dr. Mark Greene, chief resident of the ER.
• George Clooney as Dr. Doug Ross, paediatric emergency medicine fellow.
• Sherry Stringfield as Dr. Susan Lewis, second-year emergency medicine resident.
• Noah Wyle as John Carter, third year medical student.
• Eriq La Salle as Dr. Peter Benton, second-year surgical resident.

• Julianna Margulies as Carol Hathaway, nurse manager in the ER.
• Christine Harnos as Jennifer Greene, Mark Greene's wife, a law student.
• William H. Macy as Dr. David Morgenstern, chief of surgery and chief of emergency medicine.

• Holly Gagnier as Tracy Young, Doug Ross' medical student.
• Glenn Plummer as Timmy Rawlins, one of the ER desk clerks.
• Vanessa Marquez as Wendy Goldman, a nurse.
• Yvette Freeman as Haleh Adams, a nurse.
• Deezer D. as Malik McGrath, a nurse.
• Abraham Benrubi as Jerry Markovic, one of the ER desk clerks.
• Scott Jaeck as Dr. Steven Flint, chief of radiology.
• Paul Benjamin as Mr. Ervin.
• Elizabeth Ruscio as Sarah Logan.
• Michael Fairman as Mort Harris, head of the private practice visited by Greene.
• Julianna McCarthy as Mrs. Raskin, a regular patient in the ER.
• Troy Evans as Officer Martin, a police officer.
• Michael Cavanaugh as Suzanne's Dad, the father of the girl who crashed his car.
• Jeff Doucette as Mr. Larkowski.
• Christine Healy as Administrator.
• Liz Vassey as Liz.
• Tracey Ellis as Annette.
• Shiri Appleby as Ms. Murphy, a teen with ectopic pregnancy.
• Miguel Ferrer makes an uncredited cameo as Mr. Parker, the patient with possible lung cancer.

The script for this episode was written by author Michael Crichton. It is very heavily based on a screenplay he wrote in 1974, then entitled "ED" (for emergency department) based on his own experience as an emergency room intern whilst attending Harvard Medical School.[2] No producers showed any interest in his screenplay and so Crichton turned his attention to other projects. He later began a partnership with director Steven Spielberg. Spielberg read the screenplay and was intrigued, expressing interest in adapting it into a movie. However, when Crichton informed him of a book he was writing about dinosaurs, which would go on to be the 1990 novel, Jurassic Park, Spielberg shelved "ED" and worked on adapting the novel into the 1993 feature film of the same name.

After the release of Jurassic Park, Spielberg returned his attention to "ED." Upon considering the potential for there to be a series of several movies based upon the idea, he realised that it may be better as a series of television episodes. He passed the script on to a team at his production company, Amblin Entertainment. Anthony Thomopoulos, then head of Amblin's television division, contacted Les Moonves, then CEO of Warner Bros. Television and told him about the idea for the show and sent the script to them. John Wells, under contract at Amblin at the time, became attached to the project as a producer. He and Crichton discussed the project over lunch, discovering they had very similar ideas on how to approach the series.

Warner Bros. took the project to NBC, together with the 'dream team' of Crichton, Spielberg and Wells. Warren Littlefield, head of NBC Entertainment at the time, liked the project, but there was much debate and controversy among other executives at the network, who were dubious about the nature of the story as well as the number of characters and parallel subplots that they were worried audiences wouldn't be able to follow. NBC offered a chance to make a two-hour made-for-TV movie from the script, which the studio and team rejected. They then tried to get picked up at another network but were unsuccessful, before eventually returning to NBC who greenlit a pilot.[3] Wells contacted previous collaborator, Neal Baer, a qualified doctor, to update the script as a lot of the medicine had become outdated in the almost two decades that elapsed since it was written.

The pilot was greenlit only five weeks before shooting had to begin if the episode was going to be made in time to enter the fall schedule. As such, the casting process was very rushed. George Clooney was the first member of the cast to officially sign on. Clooney was under contract at Warner Bros. at the time so was only available to projects at that studio. He was being considered for roles in two separate NBC pilots. John Frank Levey, the casting director, met with Clooney who read one single scene for the part of Dr. Doug Ross. He chose the role on ER over the other pilot offered to him.

The next to be cast was Anthony Edwards as Dr. Mark Greene. He was initially reluctant to audition as he was intending to take a break from acting to focus on directing, but was convinced by his wife and agent at the time. The casting directors and executives were so impressed by his performance that no other actors read for the part, and he was cast immediately. His manager pushed back the feature film he was attached to direct in order for Edwards to be available to shoot the pilot.

Wells was impressed by actress Sherry Stringfield in NYPD Blue, and when she decided to leave that show he asked her if she would be interested in another project. Stringfield was amazed by the script and went to audition. Levey was very impressed with her audition and she was cast the same day.

Noah Wyle, just 23 at the time, was sent the script by his manager before he had the chance to tell her that he wasn't interested in television projects. Unaware that the lengthy script was actually a television pilot and not a feature film, he went to audition for the role of Dr. John Carter. Wyle's final audition was against Raphael Sbarge, the producers original choice for the role, but he won them over with his performance.

The last part to be cast was that of Dr. Peter Benton. The pilot had already begun shooting and the role had yet to be filled. None of the actors auditioned were felt to be suitable. One of these actors was Michael Beach, who would later go on to have a recurring guest role on the show as Al Boulet during seasons 2 through 6. Eriq La Salle was spotted in a pilot for another project, and was brought in for an audition. He turned up dressed in blue surgical scrubs and performed yoga in the corridor outside the casting office. The casting directors felt he was the best they had seen and therefore La Salle was cast. He then faced the challenge of learning his lines, many of which were long monologues filled with medical jargon, in less than a day before he was required on the set for shooting.

Julianna Margulies was cast as the part of nurse Carol Hathaway originally for the pilot only, after producers saw her guest appearance in another NBC show, Homicide: Life on the Street. It was not initially planned that she appear in any future episodes, as her character was actually supposed to die after the events of the episode. As such, she is credited as a special guest star rather than as part of the main cast. Her performance in the episode and her chemistry with George Clooney convinced producers that she should become a regular character. However, the producers of Homicide also wanted to offer her a regular role. Margulies chose the role on ER and therefore the following episode reveals that Carol did not die and is at home recovering. Co-star Anthony Edwards said in an interview, "I think everybody fell in love with Julianna immediately and her character and it was great that they were able to not kill her off. She was too good to kill, obviously."

Crichton's screenplay was virtually unchanged from its original version, other than the fact that Dr. Susan Lewis became a female and Dr. Peter Benton became African-American and roughly twenty minutes of content was removed in order for the episode to be shown in a two-hour slot on television including commercial breaks.[4]

The pilot was shot in the disused Linda Vista Hospital in Los Angeles, California, which had ceased operations in 1990, due to a lack of time and money necessary to construct a set in time for shooting.[5]