PHILADELPHIA — No matter what the title of the show promises, the skies over this location shoot for “It’s Always Sunny in Philadelphia” had offered only thunderstorms broken up by occasional periods of mugginess. Maybe that was typical weather for this recent summer morning, or maybe it was the influence of the authors of the scene about to be filmed, who were thousands of miles away, busy with their regularly foreboding duties.

For most episodes of “It’s Always Sunny,” the proudly depraved FX comedy about miscreant friends who run their own bar, the stars and producers Rob McElhenney, Glenn Howerton and Charlie Day keep the writing assignments to themselves and their like-minded colleagues.

But for this installment, the three (momentarily gathered beneath an overhang, waiting for the rain to pass) entrusted those duties to David Benioff and D.B. Weiss, the creators and show runners of the starkly brutal HBO fantasy “Game of Thrones.”

Benioff and Weiss have won acclaim for “Game of Thrones,” about a battle for power in a cutthroat medieval world; last week, it was nominated for 16 Emmy Awards, including best drama.

They have also gained a reputation — consistent with the George R. R. Martin novels from which the show is adapted — for killing off prominent characters in gruesome fashion.

In one of their few outside writing assignments, Weiss and Benioff wanted to split sides in the figurative sense, not requiring broadswords or battle-axes. They pursued “It’s Always Sunny” not only because they were fans of the show and its creator, McElhenney, but also because its humor and fast-paced half-hour format would challenge them.

Emailing from Belfast, where they do most of their “Game of Thrones” work, they said that McElhenney was “a sadist and likes to watch drama writers crash and burn.”

McElhenney said he embraced their offer knowing it was a good opportunity for his show, and respecting what Benioff and Weiss do on “Game of Thrones,” “executing that many story lines with that many characters, and creating such a rich and full tapestry every week.”

Also, McElhenney said, “you don’t know who’s going to get their head chopped off.”

The “Game of Thrones” creators first met McElhenney and his wife, Kaitlin Olson, another star of “It’s Always Sunny,” several months ago at a party in Los Angeles and confessed their love for the comedy. “Like all actors,” Benioff and Weiss said, “they respond well to worship.”

As it turned out, “Game of Thrones” is one of a few shows that McElhenney and Olson, who are parents of two young sons, still watch regularly. “Once those kids are in bed,” Olson said, “we get real grown up.”

When Weiss and Benioff hit upon an idea for “It’s Always Sunny” — a gloss on the novel “Flowers for Algernon,” in which Day’s ne’er-do-well character is convinced that a scientific experiment is making him smarter — they suggested it to McElhenney.

Asked whether they were tempted to kill off any principal “Sunny” characters, Weiss and Benioff said: “Nah. Those guys are immortal in our minds. They’re like the Simpsons.” As for supporting players like the unnamed Waitress or the down-on-his-luck Rickety Cricket, well — they didn’t quite answer the question.

Certainly, the “Game of Thrones” connection and the large youthful audiences it brings in are a powerful promotional opportunity for “It’s Always Sunny” at a time when the series is migrating to a new FX spinoff channel, FXX, that is aimed at younger viewers. (Both the new channel and the new season of the show will make their debuts in September.)

Still, McElhenney and his colleagues had to be sure that the “Game of Thrones” guys could emulate the wayward tone of “It’s Always Sunny.”

More crucially, McElhenney said that the intricate serialized style of “Game of Thrones” was “the polar opposite of what we do on ‘Sunny.’”

Rule No. 1 of “It’s Always Sunny,” he said, is that “our characters never really change or grow,” or else “there’d be no show.”

By design, he said, “you could pick up Episode 6 from Season 7, and have no idea what was happening in the previous seasons or episodes, and catch right up.”

So Weiss and Benioff were subjected to the same process any other potential contributor would undergo. They went to the production offices, pitched their idea to the writing staff, wrote an outline that was further refined by the show’s writers and then a script that was also revised.

The “Game of Thrones” creators said it “was humbling to step into a room with 15 guys who are smarter, funnier and quicker than we are.” They added, “They were all very gracious about the fact that we’re both brutally unfunny.”

But the writers of “It’s Always Sunny” said that Benioff and Weiss delivered a script that was structurally sound.

“We knew that most importantly there would be a working story, and we can always add or subtract jokes,” Day said. “If a random actor came in and wanted to write an episode, I would be concerned. But anyone who runs a successful show, I know they’re probably going to do a good job.”