Anyone who’s read Guru: My Days with Del Close (2005), penned by former Close student Jeff Griggs, knows that the comedian kept his cards to his chest, revealing personal info in short bursts. While Griggs chose to circumvent this difficulty by relating eccentric anecdotes from his time as Close’s assistant, Johnson faces the challenge head-on (he admits he hasn’t read Guru, for fear it would have dissuaded him from writing Funniest). He did his research—diving into archived Chicago Tribune reviews of shows Close directed at Second City and chatting with confidants like Jim Belushi, Dave Pasquesi and Murray.

The undertaking began shortly after Close died in 1999 from a lifetime of smoking, drinking and drug use. (The book’s title comes from Close’s final words: “I’m tired of being the funniest one in the room.”) Johnson idolized Close, having been a member of the first-ever iO—then Improv Olympic—Harold team, Baron’s Barracudas. A friend suggested he write a screenplay based on his teacher’s life, a task that proved difficult for Johnson, who struggled to find the point of view he wanted to take. With help from his wife, the project morphed into a biography.

“I don’t think I could have written anything when Del was alive; he didn’t really want all the truth known,” Johnson says. “Having said that, I don’t think many people would have handled the challenging, difficult, unflattering stuff with as much sensitivity as I did.”

He’s talking, of course, about the man who used to get hammered and sleep under the Tiffany Theater stage in Los Angeles, emerging in a daze the next morning just in time to teach improv workshops. Or the guy who chastised Halpern’s boyfriends until they all but sprinted the other way. Yes, he also once smoked out of an Egyptian opium pipe his friend John Brent stole from the Met. (This prompted the best late-to-rehearsal excuse ever: “I don’t feel very good. It was either the two-thousand–year-old opium I shot, or the ham-and-cheese sandwich I had at the Roachcoach.”)

And while Johnson backs up these anecdotes with eye witnesses, much of Close’s early life remains shrouded. He claims to have traveled as a teen with Dr. Dracula’s Magic Horror Show, assisting with tricks and performing a fire-eating act. As expected, records of such outfits are spotty. Then there’s the matter of his father’s suicide, a story Close told numerous times, each version slightly different. Sometimes Del Senior drank a glass of battery acid in the kitchen; sometimes they were in his dad’s office; sometimes he imbibed Drano. Journalists for the Manhattan Mercury report that Close wasn’t even in his Kansas hometown when it happened.

Johnson’s admiration for Close comes through in the retelling—he’s candid about drugs and detail discrepancies, but ultimately forgiving. “Del wanted his legend to grow; people would tell stories about him, and he wouldn’t correct them,” Johnson says. “I didn’t want to sensationalize his story to diminish the legend, but I wanted to tell the truth.”

Johnson discusses Del at iO Monday 7.