It's Always Sunny in Philadelphia It's Always Sunny in Philadelphia

In Paddy's Pub's five years of business, It's Always Sunny in Philadelphia has spawned everything from a major sports arena staple (Greenman) to a nationwide musical tour (The Nightman Cometh). With Season 6 around the corner, the question now is: What will the series' hardcore fans latch onto next?

"We never really know what's going to catch on. We wound up watching that Nightman episode from Season 4 and not really thinking it worked so well and then it just kind of took off," creator, executive producer and star Rob McElhenney tells TVGuide.com. "I had to talk Charlie [Day] into doing the Greeman thing. He didn't think it was going to work."

Watch full episodes of It's Always Sunny in Philadelphia

Whatever it is that grabs viewers, it seems to be working like a charm.The FX comedy series, which started out as a $200 YouTube video, is entering its sixth season with no detours in sight. The series has not only generated a devout community of Flipadelphia champs and McBoyle haters,  but also last year's successful Nightman

musical revue, based on the Season 4 finale."That's such an incredible payoff for all the work. Because when you're in television, you don't get a chance to see the audience," McElhenny says. "To see everybody in one room for 6,000, 7,000, sometimes 8,000 people, singing along to everything that you wrote and reciting lines as you're going along, it just makes you feel like, 'Wow, people are really watching the show."

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The success of Sunny has also opened new doors for the cast. McElhenney wrapped his arc on Lost earlier this year. Fellow executive producer and star Day is now shooting the Jennifer Aniston comedy Horrible Bosses and can currently be seen in Going the Distance."Any time you can go off and do other things and still be able to stock your creative fire elsewhere, it's going to help in this," McElhenney says. "It feels like you can come back in and you're doing something new and fresh each time, as opposed to the drudgery of writing another episode."Adds Day: "It really makes me appreciate the success of the show and the people I'm working with."

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That influence also extends to Sunny's line-up of guest stars this season, which includes Day's Going the Distance and Horrible Bosses co-star Jason Sudeikis."In one episode, we discover that there was once — sort of like a fifth Beatle — there was another member of the gang played by Jason Sudeikis," Day says. "So we go and kind of dig him back up and it makes for a pretty hilarious episode."However, the guest star to watch out for may be Tom Sizemore, who plays a crazy trucker."We were thinking the character needed to be someone who is edgy and grizzled and someone who is potentially dangerous at any moment," Day says. "He seemed excited that we wanted to work with him and that it wasn't a joke, but that we were serious about his abilities as an actor. He came in and he nailed it."Beyond Season 6, the cast members have said they would be open to another music tour. They have one year left on their contract with FX."Every once and awhile, it's great to have something special for the fans but if you're putting out another piece of product, it starts to feel a little too commercial for us," McElhenney says."We like the idea of continuing what it is that everybody loves and then, when it's something special, we'll unleash it."Despite their busy schedules, it doesn't look like The Gang is closing up shop just yet."It's tough to say but I'll continue with it as long as audiences are enjoying it sand I feel good about the product," Day says. "I don't see that going downhill anytime soon so I see life past the seventh season."It's Always Sunny in Philadelphia premieres Thursday at 10/9c on FX.