Heidi Montag and her "Hills" co-star and real-life boyfriend Spencer Pratt are not exactly novices when it comes to mugging for the camera. But, they admittedly are not used to acting, which is why they were so excited to make their sitcom debut on "How I Met Your Mother."

The couple taped a cameo for the sitcom on Wednesday -- along with Kim Kardashian -- for an episode set to air on January 12 at 8:30pm on CBS. "We are very excited to be on the show," Montag told MTV News from the set.

"It's the greatest show, besides 'The Hills,' on TV. It's an honor and privilege. I like it here," Pratt said. "This is our first cameo. We're making history."

The premise is that the reality-show couple is "featured on the cover of Them Weekly -- a [tongue-in-cheek] reference to US Weekly -- and we pop out of the magazine," Spencer explained. They then taunt the show's star, Jason Segel, over his "bathroom issues."

The cameo gave the couple an opportunity to act in front of a green screen. "Heidi did a green screen for FNMTV with Pete Wentz," explained chroma-key virgin Pratt. "It's my first green screen, so [I'm] very, very nervous! I was told not to wear green, and green's my favorite color." Montag teased, "You're green to it!"

The couple also got serious and took some time out to [article id="1599286"]share their opinions about California's controversial Proposition 8[/article]. "I'm very for equal rights," Montag said.

The show may have given the dynamic duo the exciting chance to be a "figment of your imaginations" -- and the chance to work with green-screen technology -- but it also showed them how much work goes into making a weekly sitcom. "It's way more of a production [than reality TV]," Pratt said. "We usually don't have a set. I mean, we have a director, but he doesn't direct."

"I'm a little nervous," Montag admitted. "It's a big audience that watches you here. In reality we have a couple cameras, but nobody's watching you. There is pressure [here] to nail your lines."

One thing that helped Montag with her nerves was working alongside fellow reality star Kim Kardashian. "I love Kim. She is the sweetest girl ever. So I am so glad we are on the same one as her," she said of the episode.