Last week's episode of The X-Files is one fans will, no doubt, never forget. I mean, how can any of us erase from our minds the image of Fox Mulder tripping on mushrooms and proceeding to perform a country western dance?

What you may not have realized at the time, though, was how four different, well-known pieces of music were weaved together to create a not inexpensive backdrop to that sequence — "Somethin' Bad" by Miranda Lambert (featuring Carrie Underwood), "Achy Breaky Heart" by Billy Ray Cyrus, "Honky Tonk Badonkadonk" by Trace Adkins and "Misery Is the River of the World" by Tom Waits. The first three, of course, appearing during the country western part of his trip and the latter appearing in Mulder's vision of psuedo Hell.

"Babylon" was an episode that walked a fine line between comedy and tragedy, so it was key that the music used throughout do exactly that as well, executive producer and creator Chris Carter, who penned and directed the episode, explained to Mashable later. These songs were in his brain from the beginning; when tackling episodes with a lighter touch, he say he tends to "think musically."

The choices here were not made lightly, but in the case of X-Files they never are. In fact, some of the brightest spots in the show's musical history came because someone had a clear vision for a scene, wrote it in and willed it to be so. The same is true for this miniseries, which wraps up Monday.

Securing these songs isn't always easy, though. That's when Tricia Halloran steps in.

As the music supervisor for the miniseries — she didn't work in the original series but her love for it, she assures, predates her work in the television industry — it's her job to both get the proper clearance for music choices in the scripts and fill in any blanks with picks of her own. An example of the latter: She used a song from one of her favorite blues bands, Big Jack Johnson & The Oilers, in "Founder's Mutation" during a bar scene set in Texas. It hadn't been scripted in and she thought it was just what the scene needed.

Because the use for a given piece of music varies from network to network and series to series, nailing down a particular song a writer has in mind isn't always easy — or cheap. But say you're clearing a recognizable song (see: Petula Clark's "Downtown" in "Home Again") for the most common type of clearance (meaning, according to Halloran, it will be usable across all media, all over the world, in perpetuity): a song could set you back $30,000 to $40,000 — maybe $50,000, if the artist is high profile. And if you happen to choose an up-and-coming band who needs the exposure, some songs could run as little as $3,000.

Networks generally don't like to pay over that $50,000 benchmark, Halloran says. And in cases where a song is not available for some reason, Glen Morgan, who penned this year's "Home Again," says they'll sometimes go for a soundalike. That was actually what they had to do back in 1996, when singer Johnny Mathis refused to let them use his song "Wonderful, Wonderful" for an episode titled "Home," which is notoriously one of the series' most disturbing episodes.

Regardless, it's easy to see how a sequence like Mulder's mushroom trip could quickly add up — and that's because they actually cut some songs.

"Many more songs were written in and I had to go, 'Hold on. We don't have a million dollars for music so how many do we really have to have,'" she remembers telling Carter.

Generally speaking, however, she says working on the miniseries was "a very smooth process" — not a surprise, of course, considering this is far from this team's first time at the TV rodeo.

"Some shows it's very up and down; you're replacing songs all the time and it's hard to find something that everybody likes," she says. "But on this show ... everybody respected everybody's contribution."

Paramount, too, at least to Morgan, is knowing when not to use music.

In "Home Again," the main plot is about Scully going through the emotionally draining process of saying goodbye to her dying mother, but Morgan opted not to feature any sweeping, sappy lyrical piece, instead making room for highlighting Gillian Anderson's performance.

"I just felt like Gillian Anderson is just so extraordinary I didn't want anything to get in her way," he says. "We just thought let's just back off and let Gillian do her work."

Morgan says he enjoys subverting musical expectations — a prime example being his ironic use of the sunny "Downtown" in one of the darkest scenes of his most recent episode.

"It's a compliment when people now say, 'I'll never hear 'Downtown' the same way,'" he says.

Carter admits he wasn't always a fan of using popular music on the show — doing so felt very 90210, he jokes — but he came around to the idea in Season 2. The first instance he remembers them using music in such a way was in the two-part episode that started with "Duane Barry" and kicked off the storyline about Scully's abduction.

Since then, they've stuck to the same general principle: "What I like about the way we use music is that it actually works in the episode in a way that shows thoughtfulness rather than just a knowledge of pop," he says.

There are no lyrical songs used in the finale because it's played as a thriller, and Carter says he prefers to have mythology episodes mostly composed by Mark Snow, the man behind the iconic theme. But his hope is they'll be able to all come back and play again — hopefully sooner than later.

"Of course we always want to lay it all out there, so if you lay it all out there, I think that always creates the possibility for more — and also the possibility for better," he says. "This was a big challenge to get this show up and running, catch up with fans, catch up with the casual viewer and indoctrinate the new fans. So there was a lot resting on the show's shoulders to do that, so it was not necessarily like we picked up exactly like we picked up where we left off with the greatest of ease."

He adds: "There was a lot riding on this so we wanted to put it all out there on the screen and I feel like we did that and I think it leads to, as you're going to see in episode 6, a chance to hang the episodes on a very big cliff. It creates the possibility for more. Even if there weren't more, I think it's an ending that begs questions and that's where the The X-Files really lives."