Many future big stars made guest appearances on The X-Files many years before they became household names. While the two realms are much closer in prestige nowadays, back in the 1990s when The X-Files was at its peak, acting in big-screen movies was still definitely seen as a more important gig than working on a TV show. However, everyone has to start their showbiz career somewhere, and for many, appearing in one-off guest-starring roles, bit parts, or as small supporting characters serves as that way in.
While it was one of the shows that helped ease TV dramas toward more serialized storytelling with its famous alien mythology arc, The X-Files was never still very much a "case of the week" type procedural for most episodes, leaving the door wide open for different guest stars to shine every time out. Whether those guest stars were playing the local law enforcement officers either assisting or getting in Mulder and Scully's way or the villain of the moment, ample opportunities were provided to stand out on The X-Files.
Presented below are 21 actors and actresses that appeared on The X-Files either before they were famous or before they finally landed that big role that made them a leading performer. Included among the lot are future sitcom stars, future horror icons, and even future Oscar and Emmy-winners. Here's every X-Files guest star that went on to be a big deal.
Tobin Bell
Four years before playing Jigsaw in Saw would make him a horror icon, Tobin Bell guest-starred on the season 7 X-Files episode "Brand X." He played a heavy smoker named Darryl Weaver, the only surviving participant of a deadly experiment involving engineered cigarettes.
Jack Black
Long before movies like School of Rock would make him a comedy all-star, Jack Black guest-starred on the X-Files season 3 episode "D.P.O." The story focused on a teenage outcast who gains the ability to kill people with lightning. Black played Bart 'Zero' Liquori, friend to the main character, that was until he got struck.
Bryan Cranston
A couple of years before Malcolm in the Middle and a decade before Breaking Bad, Bryan Cranston guest-starred on the X-Files season 7 episode "Drive." Cranston played Patrick Crump, a seemingly deranged man who took Mulder hostage but ended up a sympathetic figure, sadly dying in the end. The episode was written by Vince Gilligan and helped Cranston later get the Walter White role.
Michael Emerson
A few years before playing the devious Ben Linus on Lost, future Evil star Michael Emerson guest-starred on the X-Files season 9 episode "Sunshine Days." He played Oliver Martin, a man so obsessed with The Brady Bunch that he used his mental abilities to make the family materialize in his home.
Seth Green
Years before projects like the Austin Powers movies and Buffy the Vampire Slayer would make him a star, Seth Green appeared in the X-Files season 1 episode "Deep Throat." Green played a stoner kid named Emil, who gives Mulder information on a possible UFO while being treated to a meal.
Laurie Holden
Laurie Holden is the only person discussed here to have more than a guest role on The X-Files, as she recurred as Mulder informant/Syndicate member Marita Covarrubias for several seasons. While this was her first sizable role, her future turn as Andrea on The Walking Dead would make Holden a much bigger deal.
Felicity Huffman
Long before her leading, Emmy-winning role on Desperate Housewives, Felicity Huffman guest-starred in the X-Files season 1 episode "Ice." Huffman played a scientist in the episode, which was similar in plot to the classic horror movie The Thing, with a mysterious Antarctic worm taking over people's minds.
Shia LaBeouf
Long before he was arguably more infamous than famous, Shia LaBeouf had a cameo in the X-Files season 7 episode "The Goldberg Variation." LaBeouf played a sick child named Richie, whose central character, Henry Weems, was trying desperately to use his seeming power of luck to find a way to cure.
Lucy Liu
Years before she broke out on Ally McBeal or became one of Charlie's Angels, Lucy Liu guest-starred in the X-Files season 3 episode "Hell Money." The story involved an underground lottery involving human organs, and Liu plays Kim, a young woman who desperately needed expensive cancer treatment.
Donal Logue
Before starring on Grounded for Life or playing Harvey Bullock on Gotham, Donal Logue was featured in the X-Files season 1 episode "Squeeze." It was the first-ever "Monster of the Week" episode and focused on liver-eating mutant Eugene Victor Tooms. Logue played jerk FBI agent Tom Colton, who hated Mulder.
Jane Lynch
Prior to her breakout role on Glee making her known primarily as a comedic actress, Jane Lynch guest-starred on the X-Files season 9 episode "Lord of the Flies." Lynch played school principal Anne Lokensgard, who hid the dark secret that she was actually a monstrous part-human/part insect creature.
Dean Norris
Many years before high-profile roles on Breaking Bad and Stephen King's Under the Dome made him much more recognizable, Dean Norris appeared in the X-Files season 2 episode "F. Emasculata." The story concerned a deadly outbreak at a prison, with Norris playing a U.S. marshal named Tapia.
Aaron Paul
Another future Breaking Bad star, Aaron Paul, guest-starred in the same X-Files season 9 episode Jane Lynch did, "Lord of the Flies." Paul played a self-styled teenage daredevil that goes by the hilarious name of "Sky Commander Winky." He survived the episode but didn't emerge physically unscathed.
Ryan Reynolds
Decades before becoming an A-list movie star in the likes of Deadpool and Free Guy, Ryan Reynolds guest-starred in the X-Files season 3 episode "Syzygy." Reynolds played a high school jock named Jay DeBoom, who actually ended up being the opening kill of the episode, thanks to a pair of witches.
Giovanni Ribisi
Long before appearing in movies like Avatar, Saving Private Ryan, and Lost in Translation, Giovanni Ribisi guest-starred alongside Jack Black in the X-Files season 3 episode "D.P.O." Ribisi got the meatier role, playing the titular lightning-powered teenager Darin Peter Oswald.
Tony Shalhoub
While he'd gained some fame in a supporting role on Wings, Tony Shalhoub guest-starred in X-Files season 2 episode "Soft Light" many years before he would win multiple Emmys for Monk. Shalhoub played a scientist experimenting with dark matter, which resulted in his shadow itself becoming deadly.
Octavia Spencer
Long before Octavia Spencer became an A-list award-winner, she made a brief appearance on the X-Files season 7 episode "Millennium," which served as a crossover between Millennium and X-Files. Spencer showed up in the bit part of a nurse, also named Octavia.
Danny Trejo
Before roles like Machete made him a bigger name, Danny Trejo guest-starred on the X-Files season 8 episode "Redrum." The story had a sci-fi twist, as it focused on a condemned man who was caught in a time loop. As per usual, Trejo played a dangerous killer, although this time, he was stopped by Doggett and Scully.
Luke Wilson
Before movies like Idiocracy and Old School made him widely famous, Luke Wilson appeared in the hilarious season 5 episode "Bad Blood." The story was presented through the different points of view of Mulder and Scully, with Wilson playing a local sheriff that Mulder disliked but Scully appreciated.
Bradley Whitford
Years before his time on The West Wing would make Bradley Whitford a recognizable face on TV, he made a guest appearance on the X-Files season 2 episode "Firewalker." The story concerned a volcanic research team who discovered a deadly new life form, with Whitford playing team leader Dr. Daniel Trepkos.