Season 3, Episode 15
The Scot was already generating buzz for his performances in “Shallow Grave” and “Trainspotting” but was far from a recognizable name when he played a would-be robber named Duncan Stewart. When his convenience store holdup goes awry after his partner in crime and the cashier are shot, he panics and takes nurse Hathaway (Juliana Marguiles) hostage. McGregor earned an Emmy nomination for Outstanding Guest Actor In A Drama Series.
Season 9, Episode 16
Sorry, Chrisses Pratt and Hemsworth, Chris Pine is the only one to make his acting debut on “ER” In 2003 at age 23, Pine guest-starred as a drunk party boy recovering from a five-day bender stemming from a Valentine’s Day party.
Season 10, Episode 16
In her 2004 acting debut, Chastain played a 17-year-old girl who was secretly sexually abusing her father in the episode “Forgive And Forget”. The gig landed the future Oscar nominee her SAG card.
Season 5, Episode 7
Her Oscar-winning role in “The Help” was still 13 years away when Spencer played the pregnant Maria, a woman who fakes abdominal pains to get an ultrasound in order to determine the sex of her unborn child. Much to the dismay of nurse Hathaway, Maria’s ruse was all so she could abort the pregnancy if it was a girl.
Season 10, Episode 14
The Oscar-winning “Whiplash” actor appeared in a 2004 episode as Gus Loomer, a man with a medical condition who is shocked to discover he caused a fatal car accident.
Season 6, Episode 9
A six-year-old Dakota Fanning made her TV debut in a 2000 episode of “ER” as a young accident victim.
Season 9, Various Episodes
Future Oscar nominee Don Cheadle appeared in four episodes of season 9 as Paul Nathan, a medical student with Parkinson’s disease. As his disease worsens, affecting his ability to complete his surgical rotation, Dr. Corday (Laura Innes) offers to pass him if he promises to never work with surgical patients. When Nathan refuses to accept his debilitating health and end surgical work, Dr. Corday eventually fails him from the medical program.
Season 6, Episodes 7 & 9
The “Into The Wild” star had a two-episode arc as an alcoholic teen with a crush on Dr. Finch (Michael Michele).
Season 8, Episodes 11-14
Before her gig on “Mad Men”, Christina Hendricks had a four-episode arc as Abby’s (Maura Tierney) next-door neighbour Joyce, a victim of domestic abuse.
Season 1, Episode 20
Even though it was the early days of "ER", Adam Scott knew being on medical drama was a pretty big deal back in 1995. The actor played a man who was hit by a car in a season-1 episode. He later told TVLine: "It was the first season of 'ER', so it was a pretty big. Everyone was discovering it, and George Clooney was on the cover of TV Guide as this new star. I was sitting outside waiting to work and he was playing basketball with a bunch of dudes. I remember just thinking, ‘God, this dude is on the cover of TV Guide!'"
Season 6, Episode 12
14-year-old Shia LaBeouf had already made appearances on “The X-Files”, “Touched By An Angel”, and “Freaks And Geeks” when he dropped into County General as Darnel Smith, a young teen with muscular dystrophy.
Taraji P. Henson
Season 4, Episode 20 and Season 5, Episode 2
The future “Empire” star did double duty on “ER” in 1998, first appearing as Dr. Benton’s (Eriq La Salle) niece Patrice in a season-4 episode. Henson made such a good impression she was back as a totally different character in a season-5 episode.
Season 2, Episode 5
A 1995 episode featured Lucy Liu as a single mom whose son had AIDS.
A year before she earned her badge as top cop Olivia Benson on “Law & Order: SVU”, Hargitay starred as the emergency room’s front-desk clerk, Cynthia Hooper. Hooper was in a relationship with Dr. Greene (Anthony Edwards) before he eventually dumped her.
Season 10, Episode 3
Before he made his mark in “High School Musical”, a 16-year-old Zac Efron rolled up to County General as a young man with a fatal gunshot wound.
Season 9, Episode 12
Five years before landing his Emmy-winning role of Jesse Pinkman on “Breaking Bad”, Aaron Paul played a troubled stoner with a nose and chin piercing named Doug, who must choose whether to put his young siblings in foster care.
Season 4, Episode 15
Eva Mendes is another famous face who made her acting debut on “ER”. She guest-starred as a babysitter named Donna who brings her charge in for a checkup after the sick girl raises concern in the 1998 episode “Exodus”.
William H. Macy
Season 1 through 4
Before four seasons in the recurring role of Dr. David Morgenstern, chief of surgery and emergency medicine at County General, who was eventually suspended from his post after a surgical mistake. Macy returned in a guest appearance in 2009 during the series’ final season.
Season 6, Episode 13
The late “Star Trek” actor appeared in a 2000 episode as a young boy whom County General’s doctors can't save after a devastating car accident.
Season 4, Episode 1
“ER” kicked off its fourth season with a special live episode, performed twice: once for the East Coast audience and again for the West Coast. In both of those episodes, the future Ron Swanson, Nick Offerman. The “Parks And Rec” actor showed up at County General as a rocker named Rog.