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Del Close, a seminal figure in Chicago’s improvisational movement who had far-flung influence and was a priceless comic actor, died Thursday. He was 64.

Mr. Close died at Illinois Masonic Medical Center, where he was hospitalized Feb. 26 with a failing heart and respiratory system.

His gaunt, spectacled appearance lent him an avuncular air, and his humor was more the stuff of inflection and desert-dry wit. But he was a cornerstone of Chicago’s improvisational scene for decades, an important player in the Second City movement and, as years passed, a reliable actor in everything from modern plays to Shakespeare.

For instance, director Robert Falls cast Mr. Close in his celebrated “Hamlet” at Wisdom Bridge Theatre in 1985–and in the supporting role of Polonius, Mr. Close practically stole the show (and won a Jeff award) with a portrayal whose cynical wryness seemed light-years removed from the traditional interpretation.

That was Mr. Close’s great hallmark: surprise. No matter what the part, no matter how many times you saw him reach into his bag of comedic tricks, he almost always startled and delighted with originality and invention.

Born in Manhattan, Kan., Mr. Close worked off and on with Second City for years, beginning as a performer in the ’60s with co-stars including Joan Rivers. He left to co-form the Committee satirical troupe in San Francisco but returned in the ’70s as a Second City director and resident guru. At Second City, he was mentor to the late John Belushi and scores of other improvisational hopefuls. He was Second City resident director and co-director from 1973 to 1982.

But then Mr. Close branched out to explore his own theories, experimenting with completely improvised performances. (Second City traditionally rehearsed improvisationally but performed mostly set shows.)

He and fellow improvisational enthusiast Charna Halpern worked on a variety of stages together over the years, finally opening their own, the ImprovOlympic, on Clark Street near Wrigley Field, in 1995. With Kim Johnson, they co-authored the textbook “Truth in Comedy.” His work and teaching often incorporated an improv team game he invented called the Harold.

“He changed the face of improvisation,” Halpern said. Expanding on traditional techniques of brief improvisational sessions, Mr. Close’s Harold kept teams of actors going for 45 minutes, learning to develop themes and stories in their improv they would keep coming back to after moving on to other bits, encouraging teamwork even in spontaneity.

“His theory was all about loving and respecting the actors you were working with,” Halpern said. “The idea was, no one survives if everybody doesn’t.”

The Harold has been employed in improvisational performance and teaching in countries as far flung as Germany and Japan, as well as all over the U.S., including Los Angeles, where the ImprovOlympic has a branch.

In the late ’80s, Mr. Close patched up his aesthetic differences with Second City, which came to rely more heavily on his theories, and returned to directing there, staging one revue in which he fired the entire cast and replaced it with performers including the late Chris Farley.

“How can you describe Del?” mused current Second City producer Kelly Leonard. “You can’t put his importance to this work into words. As a person, he was wonderful and difficult, he was acerbic and sweet and surly. When he found out I was writing a thesis, he left a first edition relating to the topic on my desk the next day, a book worth at least $175. He would do stuff like that.”

But for all of his contributions to improv comedy, Mr. Close leaves behind a remarkable legacy as a conventional actor.

His film work includes “The Untouchables,” “The Blob” and “Ferris Bueller’s Day Off.” On stage, he performed in Steppenwolf Theatre’s “Picasso at the Lapin Agile” and “You Can’t Take It With You”; “The Time of Your Life” for Remains Theatre; and “Pal Joey,” “All’s Well That Ends Well” and “The Merchant of Venice” for the Goodman Theatre.

His directing credits include work for New York’s Lincoln Center, and for a time he co-wrote “Wasteland” for DC Comics.

In a bio for one Second City show, he wrote: “This is the first Second City revue Del has directed sober. He hopes it doesn’t show.”