Over its 15-season run,ERwas a cultural juggernaut—a hospital drama so intense it made your palms sweat as a code blue unfolded. But beyond the gurneys and the chest tubes, the show also served as a launchpad for many future A-Listers. Think of it as the sister project toLaw & Order. Almost every actor in Hollywood had a stint onLaw & Order.
And no, we aren’t just talking about how the show produced mega stars likeGeorge ClooneyandJuliana Margulies.Instead, we’re looking at the supporting acts; those who played patients or the blink-or-you’ll miss doctor. Because between future Academy Award winners to mega Marvel legends,ERwas the early testing ground where a lot of big names made small, but mighty, appearances. So scrub in and take a look at the familiar faces that entered County General Hospital.
10Aaron Paul
Long beforeBreaking Badmade him a household name,Aaron Paulpopped up in season 9 ofERas Doug, a pierced and mohawked 20-year-old called in to help care for his younger siblings after their mother lands in the ER with a bad case of pneumonia. Unfortunately, the reunion quickly spirals as his mother berates him and accuses him of being high.
Doug may have looked like the classic delinquent, but Paul’s performance brought out his emotional core, particularly during a scene where he struggles with the thought of becoming a full-time caregiver for his siblings. In just one episode, Paul displayed the raw emotional honesty that wouldlater win him Emmys. He may not have had blue meth here (at least we think), but the dramatic chops here already cooking.
9Jessica Chastain
All great actors had to start somewhere— and forJessica Chastain, that just so happened to be in season 10 ofER.Making her professional debut, Chastain played Dahlia, a devoted teen who had been taking care of her neurologically impaired father. But while she may be well-intentioned, Dahlia is clearly in over her head as her lack of medical training becomes painfully apparent amid the chaotic check-ups in the trauma rooms.
Though brief, Chastain’s performance was gripping. She packed a mountain of fear, love, and emotional conflict into just a few scenes, with her expressive eyes doing much of the heavy lifting. If anything, this minor role became a testament to her ability to command attention, hinting at the intensity she would eventually bring toher later award-winning roles.
8Chadwick Boseman
In the final season of the show,Chadwick Bosemanmade a memorable appearance as Derek Taylor, the son of a once-great boxer (played by the late Carl Weathers) who lands in the ER after his own brutal fight. Despite spending a chunk of his screen-time unconscious, Boseman radiated a quiet strength in the scenes that he had, standing toe-to-toe with Weathers and delivering a performance marked by emotional restraint and internal tension as his character tried to grapple with his failing fighting career.
Watching the future King of Wakanda hold his own against such acting royalty proved he was destined for greatness. It’s also a bit poetic that he happened to share a scene withAngela Bassett, who would go on to play his mother inBlack Panther. Ultimately, Boseman’s time inERgave audiences an early glimpse of the depth, grace, and quiet dignity that madehim so unforgettable.
7Kat Dennings
Like many long-running shows, the latter half ofERfeatured a number of bizarre plot-lines. But none were more perplexing (nor disturbing) than the one featuring a then-unknownKat Dennings.In season 12, she played Zoe Butler, a seemingly mature new love interest for Dr. Ray Barnett (Shane West). And while all appeared to be going well, the romance took a hard left turn when Zoe revealed she had lied about her age and was actually a 14-year-old girl. Gross.
Dennings walked a fine line with this performance. Her naive infatuation made audiences uncomfortable, but she also injected real emotion into the role, especially as her five-episode arc revealed a harrowing backstory of abuse at home. Yes, we may have recoiled at Zoe’s incessant pursuit of Ray, but we also grew to understand her desperation for affection. It was an early testament to Dennings' talent for blending awkward humor with empathy—something she’d later master in shows like2 Broke Girls.
Compared to the rest on this list,Josh Gad’s role is almost a mere blink-or-you’ll miss. Introduced in season 11, audiences meet him as Sgt. Bruce Larabee, a soldier working alongside Dr. Michael Gallant (Sharif Atkins) as they’re stationed in Iraq. It’s a surprisingly serious role for the man who would later voice Olaf the snowman, but Gad made sure to bring his trademark sincerity and comedic flair to the battlefield.
Whether dealing with shrapnel wounds or vehicle explosions, his cheekiness offered a much-needed moment of levity amid the scenes' intensity. Fun fact: this was Gad’sfirst Screen Actors Guild joband his first time playing a doctor—though, as he has since joked, probably his last after a technical advisor critiqued his poor bagging skills.
5Taraji P. Henson
It’s no surprise that theLaw & Orderuniverse likes to recycle actors over the years. So long as there’s a big enough buffer time between episodes, it doesn’t matter—you could play 6 different characters for all they care. And while this wasn’t common inER, it funnily enough happened to a youngTaraji P. Henson, who played not one, but two characters onER.
She first appeared in season 4 as the niece of Dr. Peter Benton (Eriq La Salle). Though only on-screen for about 30 seconds, her classic charm shines throughout. Her second appearance took a sharp turn, occurring only a few episodes later in season 5 as a young woman entangled with a Chicago street gang. It was this role that offered the first real taste of the charisma, fire, and confidence that would define many of Henson’s later roles. Even in her early career, she carried an undeniable spark, transitioning effortlessly from charm to edge.
4Jeffrey Dean Morgan
Yes,Grey’s Anatomywas not the first timeJeffrey Dean Morganplayed the role of a severely ill patient. Before Denny Duqette (or John Winchester) graced our screens, Morgan appeared as Firefighter Lorcan inER’s season 7 train disaster episode—a classic trauma nightmare with crushed limbs, ticking time-bombs, and gut-wrenching choices. Pinned under a fallen train, Lorcan faces the impossible: double amputation or death. Enter Dr. John Carter, who’s forced to do the procedure on-site.
For a single-episode appearance, Morgan delivers a powerhouse performance balancing fear, grit, and grace as he accepts his unfortunate fate. It’s haunting stuff and mere proof that Morgan could inject gravitas into any role, no matter how short-lived it may be.
3Mariska Hargitay
It’s almost comical that most people don’t remember a time whenMariska Hargitaywasn’t onLaw & Order: SVU, but alas, here we are because, before she became the face of one oftelevision’s longest-running shows, Hargitay had a relatively important role in season four ofER.Playing Cynthia Hooper, she was introduced as the wide-eyed, but often frazzled desk clerk, who was completely inept for the job.
She even ended up having a romantic arc with Dr. Mark Greene; however, that quickly got messy and one-sided, with Mark too emotionally unavailable to truly return her affection. And while Cynthia could’ve been boxed-in as the “irritable clinger”, Hargitay imbued her with a genuine sincerity that made audiences sympathize with her heartache. At least Cynthia got the last laugh by shutting down Mark after he tried to rekindle things. It’s a nice reclaiming of dignity— an early nugget of strength that’s later infused into Olivia Benson.
2Adam Scott
Before receivingan Emmy nomination in the same categoryas Noah Wyle,Adam Scottjust so happened to share a few scenes with him in the very first season ofER. Playing the role of David—a patient brought in after getting hit by a car—Scott’s character gets caught in the middle of a med-student rivalry as both John Carter and Deb Chen (Ming-Na Wen) gun for more trauma cases.
Stuck on a gurney with a massive laceration across his chest, David has no other choice than to succumb to being their guinea-pig as they race to make the most stiches. It’s a brief appearance, but Scott manages to do so much. No lines—just deadpan reactions of wide eyes and subtle glances. It’s pure comedic gold and an early example of the quiet, observational humor he’d master inParks and Recreation(and maybeSeverance?)
In season two,Lucy Liuappeared in a number of episodes as Mei-Sun Leow, a young mother desperately seeking care for her son who was dying from AIDs. What follows is a devastating arc. Her son undergoes a traumatic, botched test; she’s told the illness is terminal; and she ultimately decides to take him home to die peacefully.
It’s absolutely gut-wrenching and Liu’s performance (accent and all) is profoundly moving. She plays grief with heartache and a quiet strength, holding her own in scenes opposite the powerhouse that is George Clooney. It’s a reminder that Liu’s dramatic range extended far beyond the action or martial artsroles that made her famous, like inCharlie’s AngelsorKill Bill. Though it is quite funny that this empathetic role was soon followed by her break-out gig inAlly McBeal.Ling Woo was anythingbutthe doe-eyed nice girl.
