The future Buffy star played a helpful stoner in The X-Files' second episode.

Before becoming Mr. Courtney Stodden, the Green Mile actor appeared as a liver-eating mutant with the ability to stretch his body in two episodes.

The Gotham star played a colleague of Scully's who didn't approve of her association with "Spooky" Mulder.

The Desperate Housewives star appeared in Season 1's rip-off of The Thing as a toxicologist infected with rage-inducing parasitic worms.

Before wreaking havoc on Supernatural, Sheppard played a murderous arsonist with the ability to conjure fire out of thin air.

The West Wing star went full Rambo in The X-Files episode "Firewalker" to stop a deadly fungus.

The Lost alum played three separate characters on The X-Files. He first appeared as a philandering police sergeant, again as an FBI agent in "Fight for the Future"and finally, a super-soldier known as The Shadow Man.

The Breaking Bad star teamed up with Mulder to track down an escaped convict with contagious exploding pustules.

The Monk star played a man who was literally afraid of his own shadow. And for good reason — it could actually kill!

In Season 3's "D.P.O." Ribisi played a murderous outcast with power over electricity. Black played Ribisi's only friend, who winds up being electrocuted after he's suspected of ratting out Ribisi.

The Sex and the City star appeared in two episodes, Season 3's "The Walker" and "The Goldberg Variation" with Shia LaBeouf.

Before boarding Serenity on Firefly, Staite played the victim of a kidnapping.

Future stud-muffin Reynolds was a slightly chubby teenager murdered by two psychically connected girls in "Syzygy."

The famous crooner appeared uncredited in two episodes of The X-Files. He later said he was kicked off set for stealing a hot dog.

Everyone's favorite SVU psychologist once played a bad cop on The X-Files. Benson would not approve.

In the same episode B.D. Wong appeared, Liu also popped up in a tiny guest role as a sick woman.

Wilson appeared as a buck-toothed sheriff in one of the most beloved episodes of all time, Season 5's "Bad Blood."

Before The Walking Dead, X-Files fans knew Holden as Syndicate agent Marita Covarrubias, who appeared in 10 episodes.

Before Breaking Bad, Cranston proved his ability to play likable bad guys as a sympathetic bigot in "Drive."

In "Hungry," the Lost alum played an aspiring blackmailer with an unforgettable name: Derwood Spinks.

In a blink-and-you'll-miss-it appearance, Spencer played a nurse named Octavia.

If you ever long for the days when LaBeouf was a lovable child actor, just watch "The Goldberg Variation," in which the actor plays a bedridden boy with liver problems.

In the episode "Fight Club," the comedienne got to play identical twins, Lulu and Betty, who cause violent reactions when they're near each other.

Before playing Papa Pope on Scandal, Morton played a man who began experiencing time backward after his wife is murdered.

Scandal's First Lady appeared as a lawyer in The X-Files' eighth season.

The Firefly star had a recurring role as Rohrer, a Syndicate hybrid soldier.

Yet another Breaking Bad star, Paul appeared in "Lord of the Flies" as a rowdy teenager who auditioned for a Jackass rip-off Dumbass.

Appearing in the same episode as Aaron Paul, the Glee starred played an insect woman with the ability to shoot webbing from her mouth.

Before Lost, Emerson starred in one of The X-Files' final episodes as a man with psychokinetic powers and an obsession with The Brady Bunch.

Though not a guest star, Gilligan deserves some acknowledgement. Before creating Breaking Bad, Gilligan was a writer and producer on The X-Files and The Lone Gunmen.And thank god. It was through The X-Files that Gilligan first worked with Breaking Bad stars Bryan Cranston, Aaron Paul and Dean Norris.