1. Zac Efron NBC Episode: Season 10, Episode 3, "Dear Abby"Role: Before his singing, dancing, High School Musical days, Efron played Bobby Neville, a young boy suffering from a gunshot wound. 2. Ariel Winter NBC Episode: Season 15, Episode 15, "The Family Man" through Episode 21, "I Feel Good"Role: Before making her mark as Alex Dunphy on Modern Family, Winter had a several-episode arc on ER as a girl named Lucy whose mother has to undergo emergency transplant surgery after they crashed into an ambulance. 3. Shia LaBeouf NBC Episode: Season 6, Episode 12, "Abby Road"Role: Young, pre-Even Stevens Shia played Darnel Smith, a child suffering from muscular dystrophy, and his performance is a real tearjerker. 4. Eva Mendes NBC Episode: Season 3, Episode 15, "Exodus"Role: Ages before starring in films like Training Day, Hitch, and The Place Beyond the Pines (and also before making a baby with Ryan Gosling), Mendes played a good-hearted babysitter to a little girl with an E.coli infection. 5. Josh Radnor NBC Episode: Season 9, Episode 17: "The Advocate"Role: The future How I Met Your Mother star played a man name Keith, who was in the ER, being treated for an allergic reaction to penicillin. 6. Chris Pine NBC Episode: Season 9, Episode 16, "A Thousand Cranes"Role: Before his role as captain of the USS Enterprise, Pine had a small role as Levine, a drunk dude who visits the emergency room. 7. Rooney Mara NBC Episode: Season 15, Episode 19, "Old Times" and Episode 21, "I Feel Good"Role: Before starring in The Girl with the Dragon Tattoo and other major films, Mara had a small part on ER as Megan, a young mother who struggles with giving up her baby. 8. Gabrielle Union NBC Episode: Season 6, Episode 10: "Family Matters"Role: Before she became Mary Jane, Union played a high school student named Tamara Davis who drove into a telephone pole. 9. Chadwick Boseman NBC Episode: Season 15, Episode 6, "Oh, Brother"Role: Long before saving the world with his fellow Avengers as Black Panther, Boseman played Derek, a boxer who has to make a trip to the emergency room after a rough fight. And fun fact: his father is played in this episode by actor Carl Weathers, who famously played boxer Apollo Creed in the Rocky movies! 10. Lucy Liu NBC Episode: Season 2, Episode 3, "Do One, Teach One, Kill One"Role: A few years before making her mark as Ling Woo on Ally McBeal (and later on as one of Charlie's Angels), Liu played the mother of a 4-year-old boy with AIDS. 11. Ewan McGregor NBC Episode: Season 3, Episode 15, "The Long Way Around"Role: Not long after his breakout role in Trainspotting, McGregor played a shaggy-haired liquor store robber. 12. Josh Hutcherson NBC Episode: Season 9, Episode 8: First SnowfallRole: Long before running around with Katniss in the Hunger Games franchise, Hutcherson played Matt, a young boy who was run over by a drunk driver while building a snowman. 13. Adam Scott NBC Episode: Season 1, Episode 20, "Full Moon, Saturday Night"Role: Over a decade before he was making everyone laugh as Ben on Parks and Recreation (and a lot of other great stuff), Scott played a patient named David who had been hit by a car and suffered a super bad gash on his chest. 14. Kirsten Dunst NBC Episode: Season 3, Episode 5, "Ghosts" plus five more episodes sprinkled through Episode 22, "One More for the Road"Role: Okay, to be fair, Dunst had already made her mark as a child actor in films like Interview with the Vampire and Little Women prior to her stint on ER, but she played a homeless teen named Charlie who strikes up a friendship with Dr. Ross (George Clooney) and was still years away from starring in the O.G. Spider-Man film franchise. 15. Nick Offerman NBC Episode: Season 4, Episode 1, "Ambush"Role: Over a decade before he played Ron Swanson on Parks and Rec, Offerman played a guy named Rog whose bandmate, and "best damn bass-player" he knows, Boz is rushed into the emergency room. 16. Kat Dennings NBC Episode: Season 12, Episode 5, "Wake Up" through Episode 12, "Split Decision"Role: Before some of her bigger roles in The House Bunny, Nick and Norah's Infinite Playlist, and 2 Broke Girls, Dennings played Zoe Butler, a girl who hooks up with Dr. Barnett (Shane West) and reveals that she's actually 14 years old. 17. Kristin Davis NBC Episode: Season 1, Episode 13, "Luck of the Draw"Role: Before her days drinking cosmos and wearing Jimmy Choos on Sex and the City, Davis played a woman who accompanies her niece to the ER because of a hamster bite — and, of course, she hits on George Clooney. Because, duh, George Clooney. 18. Tobin Bell NBC Episode: Season 1, Episode 2, "Day One"Role: About a decade before traumatizing victims as Jigsaw in the Saw movie franchise, Bell threw his weight around as a hospital administrator at Country General. 19. Cara Buono NBC Episode: Season 15, Episode 22, "And in the End"Role: Way before playing Mike and Nancy's mom on Stranger Things, Buono got to play a brand new mom in the series finale of ER. 20. John Hawkes NBC Episode: Season 4, Episode 1, "Ambush"Role: Loooong before his turn as an Academy Award-nominated movie star, Hawkes played a production assistant helping film a documentary at Country General. 21. Deborah Ann Woll NBC Episode: Season 14, Episode 14, "Owner of a Broken Heart"Role: Before sinking her teeth into the role of Jessica on True Blood, Woll played a young nurse named Aurora Quill. 22. Stana Katic NBC Episode: Season 12, Episode 4: "Blame it on the Rain" and Episode 5, "Wake Up"Role: You may know her best as the co-star of Castle, but before then Katic played a patient who was in a coma for several years, but then suddenly woke up in the emergency room. 23. Michael Cudlitz NBC Episode: Season 2, Episode 16: "The Healers"Role: Long before portraying Abraham on The Walking Dead, Cudlitz played an injured fireman in a 1996 episode of ER. 24. Eric Stonestreet NBC Episode: Season 7, Episode 3Role: Another eventual Modern Family star, Stonestreet played a patient named Willie, who found himself in the ER after slicing up his ears, trying to make them look like Spock from Star Trek. 25. Josh Peck NBC Episode: Season 7, Episode 13: "Thy Will Be Done"Role: Before he became Josh, of Drake & Josh fame, Peck played a teen named Nick, visiting the ER in need of a heart transplant. 26. Aaron Paul NBC Episode: Season 9, Episode 12: "A Saint in the City"Role: Breaking Bad's Jesse Pinkman played Doug, an edgy teen who is responsible for taking care of his younger siblings after his mother falls ill.