Wallace Shawn and André Gregory may be the most renowned playwright-director duo in New York theater, so much so that two of the city’s most renowned theaters, the Public and Theater for a New Audience, are sponsoring a series to celebrate their 40-year collaboration.

Yet for many theatergoers, this will be not a retrospective but something new. Mr. Shawn, 69, is less known for his plays than for his droll roles in dozens of movies, most notably “Manhattan” and “The Princess Bride.” In all these years, he and Mr. Gregory have collaborated on just five plays, only three of which were ever formally staged. “The Wallace Shawn-André Gregory Project,” as the series is grandly called, consists of just two of them, “The Designated Mourner,” previously seen in New York by only several hundred people, and “Grasses of a Thousand Colors,” making its American debut.

Such sparseness is very much a result of the way they work. Mr. Gregory, 79, a pioneer of avant-garde theater, often rehearses a play for months, even years. He and his stable of actors (including Mr. Shawn) worked on their adaptation of Ibsen’s “Master Builder,” off and on, for 14 years, ultimately performing it in an art gallery before invited guests, never more than 20 at a time. (Jonathan Demme recently directed a film of the work, just as in 1994 Louis Malle directed a film of their long-gestating version of Chekhov’s “Uncle Vanya” under the title “Vanya on 42nd Street.”)

“I so love the process, I could work forever on a play and wouldn’t blow my brains out if it never got finished,” Mr. Gregory said one recent afternoon.