Not long ago, in an attempt to introduce my boyfriend to the brilliance that is The X-Files, I dug out some old VHS copies of episodes and showed him one of my favourite episodes. Part way through, one of the actors playing a bit part (the character committed suicide pretty early on) started to look familiar. Really, really familiar. Neither of us could quite place him for a long, long time.

Then, finally, it clicked. It was Ryan Reynolds. A very young Ryan Reynolds, but Ryan Reynolds nonetheless. Given that both I and my boyfriend are Reynolds fans (ever since Blade Trinity!) we were surprised we hadn't known he'd ever been in The X-Files. But then, between one thing and another, there are a lot of people we either didn't know or had forgotten showed up in The X-Files. So here are 10 famous actors whom you either didn't know, or might have forgotten, were guest stars at one point or another...

Ryan Reynolds
Reynolds plays Jay "Boom" DeBoom (I had to look that name up; it's wonderfully improbable!) in Syzygy (season 3, episode 13). Rewatching this episode now, it's not quite as mind-blowingly amazing as I remembered, but I still do love it silly amounts.

Seth Green
Seth Green played Emil in the second episode of The X-Files, Deep Throat. I'm willing to bet that he's the first person you thought of when you saw the title of this article; he seems to be one of the more known-about people you didn't know about. (Um, pretend that made sense.)

Mark Sheppard
You might not recognise Mark Sheppard's name, but if you've ever watched TV you'll probably recognise his face. Mark played Bob, the caretaker, in the episode Fire (season 1, episode 11). Bob has the gift of, erm, being able to set things on fire. Without matches. I also love this episode an unhealthy amount.

Shia LaBeouf
The Beef shows up quite late into The X-Files, in season 7 episode 6, The Goldberg Variation - which was sort of like Final Destination, but backwards.

Giovanni Ribisi & Jack Black
These two had already done quite a lot of film/TV work before showing up in D.P.O (season 3, episode 3) as punk-loving teenagers, one of whom seems to have harnessed the power of lightning. This might be the only episode of The X-Files to have a rock soundtrack (though I'm willing to be proven wrong by anyone with a better memory for music than me.)

Felicity Huffman
Long, long before she was a Desperate Housewife, Felicity Huffman played Dr Nancy Silva in Ice (season 1 episode 7). That's the one that's really, really similar to The Thing.

Jewel Staite
Yet another Whedon alumni who started off in The X-Files: Jewel played Amy Jacobs in Oubliette (season 3 episode 8), a girl who gets kidnapped and stuck in, er, an oubliette ("a place you put people to forget about them", as defined in Labyrinth) but whose experiences are psychically shared by a woman on the outside.

Luke Wilson
Another famous guest star who was in another one of my all-time favourite episodes - Luke Wilson, the slightly less famous brother, was in Bad Blood (season 5, episode 12). He's the sheriff who's either devastatingly attractive or, er, less attractively bucktoothed, depending on whether you trust Mulder's version of events or Scully's. Awesome.

Lucy Liu
Finally, Lucy Liu - probably best known for being in Kill Bill, now, but she'll always be Ling to me - was in Hell Money (season 3 episode 19). I'm ashamed to say that despite looking up the IMDB and Wikipedia entries for this episode, I can remember virtually nothing about it... which is probably a good excuse to finally invest in some X-Files DVDs to replace the old VHS copies.

... Also, I just said "finally", but while researching this article I dug up lots more, so there'll be a follow-up list of another 10 people you probably didn't know were in The X-Files soon.