Man-children are something of a specialty for both Will Ferrell and John C. Reilly, who have perfected the art of playing oafish yet lovable duos in films like “Step Brothers” and “Talladega Nights: The Ballad of Ricky Bobby.” They return to type this Christmas, but with a wrinkle: This time, these idiots happen to be geniuses.

In “Holmes & Watson,” Ferrell and Reilly bring their manic, absurdist spin to the frequently revisited roles of Sherlock Holmes and his partner, Dr. Watson, as they bumble their way through a case that requires them to defend the queen from the dastardly Moriarty. “We sat down and said, ‘What are the most neglected characters in literature?’” Reilly said with a laugh in a joint phone interview with Ferrell in October. “‘What hasn’t been done?’ The world needed this.”

They also spoke about their love for the famous sleuths and reminisced about their friendship of more than a decade, riffing off each other effortlessly. These are edited excerpts from the conversation.