The first rule of making a realistic medical drama is morbidly simple: patients must die. It’s become a convention of the genre, but a quarter-century ago, the idea of even minor television characters regularly croaking scared stodgy networks to, well, death. “The doctors have to save everybody!” producer John Wells remembers hearing back then. “I’m like, ‘Yeah, it doesn’t happen that way.’”

This was the world into which ER arrived on September 19, 1994. The show’s verisimilitude nearly stopped it from ever getting made. In the end, however, it’s also what made it revolutionary.

“You know somehow if you’re watching something that you feel is real, and something where the artifice is apparent,” says Wells, the original executive producer and first showrunner of ER. “That was the major decision that we made early on, that we weren’t gonna do anything that wouldn’t actually really happen. Sometimes it made for a less compelling, in a standard narrative sense, kind of a story. But I think it made for really good, exciting television that people hadn’t seen before.”

Long before “prestige drama” was a thing, ER had all the qualities of one. It was created by one of the world’s most popular authors, shepherded by Hollywood’s most powerful director, and bolstered by a diverse cast filled with stars, up-and-comers, and a future Academy Award–winning leading man. The series featured complex characters and through its story lines boldly tackled dozens of issues of the day, including racism, AIDS, substance use disorders, and domestic violence. Its influence can still be seen in the hundreds of uncompromising hour-long shows that have followed.

“You do not have to talk down to your audience. And in fact, if you don’t talk down to your audience, they appreciate it and reward you.” —John Wells, ER executive producer and showrunner

“The audience is intelligent,” Wells says. “And particularly video literate, in that they’ve seen hundreds and hundreds of hours of storytelling and they can follow. You do not have to talk down to your audience. And in fact, if you don’t talk down to your audience, they appreciate it and reward you.”

ER’s origins can be traced back to the early 1970s, when Michael Crichton was a bestselling science-fiction novelist and filmmaker. After making his directorial debut with the futuristic theme park thriller (and HBO series source material) Westworld, the Harvard Medical School graduate wrote a script based on his experiences in the emergency room at Massachusetts General Hospital. “Nobody would make the movie,” Crichton, whose nonfiction book on the same topic, Five Patients, was published in 1970, once said. Studios, he added, found it “too technical, too chaotic, and too fast-moving. It sat on the shelf for the next 19 years.”

In the 1980s, Crichton brought the screenplay to Steven Spielberg, whom he’d befriended a decade prior when they met at Universal Studios. The director liked it, but got distracted when the writer told him that he was working on a book about resurrecting dinosaurs. “I got him to open up and tell me the whole bloody story,” Spielberg told the Los Angeles Times in 1994. “And by the end of the couple of hours, we hadn’t discussed ER at all—we had only discussed Jurassic Park. I was just so in love with this story of his that we agreed on a handshake that if I would physically direct the movie, he would give me the rights to the book.”

Crichton’s Jurassic Park reached bookstores in 1990; three years later, Spielberg’s movie adaptation hit theaters and grossed more than $1 billion worldwide. After that, Crichton and Spielberg’s attachment to any project going forward made it hyperattractive to the entertainment industry. Around that same time, Tony Thomopoulos, the president of the TV division of Spielberg’s production company, Amblin Entertainment, reportedly unearthed the script for ER and told his boss that it would make for a good TV pilot. Also keen on the idea, agent Tony Krantz sent the screenplay to Wells, who had just come off making ABC’s Vietnam War–set medical drama, China Beach.

“Do you think there’s a series in it?” Wells remembers Krantz asking him. When he read the script, he loved the way it made him feel—like he’d been dropped directly into the daily chaos of a big-city hospital. So, after meeting Crichton and agreeing that the potential show should be as true-to-life as possible, ER went on the market. But, inexplicably, it didn’t sell.

“Everybody passed,” Wells says, though he makes sure to also note how low his odds were due to the television landscape at the time: “There were only four networks to take it to at that point.”

Eventually, though, NBC bought it as a two-hour backdoor pilot—a Spielberg and Crichton collaboration proved to be too tough to pass up. “They felt Jurassic Park was out just then and so with Steven and Michael’s name on it, that at least somebody would show up to watch the pilot,” the producer says. “But they didn’t have a lot of confidence in the fact that it would be a series. Which was sort of great because they didn’t pay much attention to us when we were making it.”

Even with power brokers like Spielberg behind ER, those involved in making the show weren’t overly optimistic about its chances of survival. “You have no expectations when you do a pilot,” says original cast member Sherry Stringfield, who played Dr. Susan Lewis. “You could be working with Michael Crichton, Steven Spielberg, and you’re like, ‘It may not go.’ That’s how brutal TV is. Quality can mean nothing, as we know.”

For pre-production research, Wells spent time at University of Southern California Medical Center and Cook County Hospital in Chicago. ER ended up being set in the Windy City when the original location, Boston, where NBC’s hospital drama St. Elsewhere had taken place in the ’80s, was nixed. When it came time to cast the show, Wells says the producers of ER asked themselves, “What’s it look like? And then we cast accordingly. ’Cause we wanted it to feel like it was a real place. So we did that with the patients. We did it with languages, gender, we tried to do it ethnically and racially, who comes through and who works at an urban hospital in Chicago. It’s certainly what I saw in the hospitals I hung out in.”

Wells says that Anthony Edwards, whose character, Mark Greene, is the moral center of the show, was only able to come on board because production on a movie he was supposed to be in got pushed back. The actor, of Revenge of the Nerds and Top Gun fame, played a character who was unlike most TV doctors: He actually appeared to be vulnerable. “When the character’s worked for 36 hours, he’s prepared to be the person that we all are when we’ve been up for 36 hours,” Wells says. “It’s not your best self. And to really acknowledge the difficulties of those jobs. The emotional pressures, the family pressures. And the tremendous weight that you sort of overcome as you proceed in your career.”

Noah Wyle, who played med school student John Carter, was the youngest person in the main cast. “Twenty-[three] years old and just came in fantastic and got better, every single audition he was better than the audition before,” Wells says. “Went from waiting tables to being on the cover of Newsweek six months later.” Stringfield, who had just left Steven Bochco’s acclaimed NYPD Blue, recalls reading the script for the pilot in early 1994 while watching the Olympics with a friend in her home state of Colorado. At the time, she was about to move to England—but her plans suddenly changed. “I was like, ‘This is so cool,’” Stringfield says. “I just fell in love with it. … I’d never read a script like that before. I’d never been offered a role like that before. And so literally it was like, ‘OK, buckle up.’”

Due to a scheduling conflict, Wells recalls, Eriq La Salle wasn’t cast as Peter Benton until four days into shooting the pilot. The supremely confident surgeon is the only lead character who’s African American. “An unapologetic character,” Wells says. “Benton knew that he was the best, and knew that he carried additional weight because of his skin color and just refused to be anything other than who he was. He was not about to be humble or suffer fools gladly. That was a really brave choice. It was in the script, but, man, [La Salle] went after it. And I like to think it was an important role, obviously for him, but also an important role for what could be on TV.”

And then there’s George Clooney, who played pediatrician Doug Ross, an occasionally volatile womanizer who also went the extra mile for his patients. Clooney, who had played small parts in dozens of TV shows like Roseanne and The Facts of Life, wasn’t yet a star, but it didn’t take long for him to be singled out as ER’s resident heartthrob. However, “he was never that person saying, ‘I need a bigger trailer because I’m a star now’ or anything,” Wells says. “He was always a part of the ensemble. He worked really hard to be a part of that ensemble. He understood what that meant.”

To ensure realism, there were always physicians on set and in the writers’ room. In the first years of the series, doctors Lance Gentile, Neal Baer, and Joe Sachs all made major contributions. In addition, many of the show’s extras were real-life doctors and nurses. “Everybody knew what they were doing,” Wells says. “What we did with every medical scene was that the doctors on set and then the real practitioners would show us exactly what they would do in that situation. And then we just shot it documentary style as if we were around it. We didn’t try to manipulate what we did.”

“Noah Wyle would still give you the best suturing job you’ve ever had in your life. He really took it seriously.” —John Wells

The cast of ER went through its own rigorous training. Some was physical: “Noah Wyle would still give you the best suturing job you’ve ever had in your life,” Wells says. “He really took it seriously.” On the other hand, some was mental: “George and Anthony and Eriq would get in these contests, you know, about who ended up having to say the longest, most impenetrable hunks of medical dialogue. And Eriq usually won because the surgeons have the densest of the medical gibberish.”

Looking back on it, Stringfield is thankful she was young and naïve about how hard it actually would be to pronounce intricate medical terminology. Otherwise, filming a single scene of ER might’ve been impossibly intimidating. “Literally, Eriq La Salle had a day of 70 takes—on one word,” she says. “We were crying. Then none of us had a straight face. We could be here for a week. We don’t know what this stuff is. This is truly a nightmare. You would just bobble a word. We all had those days.”

Directed by Rod Holcomb, the 88-minute pilot was shot at L.A.’s Linda Vista Hospital. There was, it turned out, a major change to Crichton’s script: Julianna Margulies’s character, Carol Hathaway, was meant to die by suicide at the end of “24 Hours,” but, Wells says, “she and George sort of had extraordinary chemistry together.” So, Carol survived. “We resurrected her from the dead, much to Michael’s chagrin,” Wells says. “He was like, ‘You know you can’t really survive what we had happen to her.’ So we changed a few lines and had her come back.”

The most iconic moment of the episode comes when an exhausted Benton, after a particularly difficult surgery, walks through a hallway and then dramatically pumps his fist.

Wells says that La Salle came up with that himself—as soon as Holcomb saw him do it, the director said, “That’s going in the credits.”

Still, Wells says that NBC wasn’t thrilled with what they saw. “They didn’t even give us notes they hated it so much,” he recalls. But Wells credits several young executives, including Kevin Reilly, David Nevins, and John Landgraf, with championing ER. All three—Reilly (WarnerMedia), Nevins (CBS), and Landgraf (FX)—have gone on to run companies of their own.

After the pilot tested well, the network finally began to realize what it had. The series premiere aired on a Monday night but subsequent episodes were moved to Thursday to go up against David E. Kelley’s Chicago Hope, a medical show that was receiving even more hype than ER. In the summer of 1994, Stringfield began noticing that every commercial on NBC seemed to be for ER. About 23.8 million people tuned in for the pilot. In 1994-95, ER’s ratings dwarfed Chicago Hope’s; the show then spent the next 10 years near or at the top of the Nielsen ratings.

“It was a different era,” Stringfield says. “There was a Thursday night lineup and people watched it. That’s why everyone was watching ER. Those shows had so much energy. People were watching them at the same time.”

If there’s one episode in the first season that showcases what ER did best, it’s “Love’s Labor Lost.” In it, a woman played by Colleen Flynn dies after giving birth, partially due to Dr. Greene’s mistakes. Bradley Whitford delivers an excruciatingly good performance as her agonized husband, and both writer Lance Gentile and director Mimi Leder won Emmys for the episode. The idea, Wells remembers, came from the producers asking Gentile what his worst nightmare was as an ER physician.

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“It was a big deal with the network,” Wells says. “Like, ‘Wait a second, the woman dies? Are you out of your mind?’ But you know, people die in hospitals. That’s what happens. That’s the place where most people go for the last moments of their lives. So we didn’t feel we could accurately show the pressures or the realities of a hospital, nor would the audience believe it, if we didn’t actually have people die there.”

As the years passed, the main cast of ER changed several times, while dozens of A-list guest stars—from William H. Macy to Mariska Hargitay to Rosemary Clooney to Kirsten Dunst to Sally Field to Alan Alda to Don Cheadle—came and went. The series, which is now bingeable on Hulu, managed to last 15 seasons and 331 episodes. Its finale aired on April 2, 2009, five months after its creator, Michael Crichton, died of cancer at age 66.

By then, Wells knew it was time for ER to end. Still, he points out, NBC didn’t try to rush things along or drastically tinker with the show before it signed off. “We didn’t have to blow it up or tear it down,” says Wells, who spent a half-decade as ER’s showrunner and remained involved in its production for all of its run. “We just walked away. The feeling has always been: If you turn down the right street in Chicago, you’d find that hospital still there.”