What’s Funny About John Mulaney and Simon Rich’s Friendship? Everything.

The comedian John Mulaney joined the writing staff of “Saturday Night Live” in 2008. On his first day, he began working with Simon Rich, a boy-wonder Harvard grad who had already published two collections of short fiction. Their first sketch, “Cash for Silver,” a werewolf-accented parody of the cash-for-gold cable ads made it to the dress rehearsal, but was bumped before it aired.

Sixteen years later, over English breakfast skillets and matching double espressos in Midtown Manhattan, Mulaney, 42, and Rich, 40, could still recite every line.

“Send it! Don’t wait! Don’t ask!” Rich said excitedly.

“Send me silver right now!” Mulaney continued.

This could have gone on for a while. But they had rehearsal to get to.

Mulaney is starring in “All In: Comedy About Love,” a collection of Rich’s prose pieces lightly adapted for Broadway that opens at the Hudson Theater on Dec. 22. Renée Elise Goldsberry, Richard Kind and Fred Armisen join him in the inaugural cast. (Armisen, another “S.N.L.” veteran, had been cast as a boatman in “Cash for Silver.”) As the run continues, performers including Lin-Manuel Miranda, Jimmy Fallon, Aidy Bryant, Nick Kroll and Tim Meadows will tap in.

The production is the brainchild of Alex Timbers (“American Utopia,” “The Pee-wee Herman Show”), a prolific Broadway director with an affinity for theater that borders other genres — the concert, the stand-up act. (He also directed “Oh, Hello on Broadway,” which starred Mulaney and Kroll.) Timbers had read all of Rich’s collections during the pandemic lockdown. During a get-to-know-you meeting in February, Timbers suggested that Rich’s short stories, many of them about relationships, marriage and heartbreak and first published in The New Yorker, could make something like a play.

From left, Richard Kind, Renée Elise Goldsberry, Mulaney and Fred Armisen are the inaugural cast. With scripts in hand, they are performing a sort of “Love Letters” for people who think they’re too hip for “Love Letters.” Credit…Sara Krulwich/The New York Times