Last weekend, Stephen Sondheim, 86, demonstrated once again that he is a showman to the core. Speaking to a paying audience at the Glimmerglass Festival between matinee and evening performances of his “Sweeney Todd,” he let drop the projected opening date of his next musical, which has been under discussion for several years: it will open, he said, in 2017, at the Public Theater in New York.

The Public Theater confirmed that the project was in development, but said that no set date has been confirmed for its performance.

The performance date may be news, but the subject is well known. Written with the playwright David Ives (“Venus in Fur“), the piece is based on two films by Luis Buñuel, “The Discreet Charm of the Bourgeoisie” and “The Exterminating Angel.” The first, in Sondheim’s précis, is about a group of people trying to have dinner together, and the second is about people having dinner together who for some reason can’t leave.

It will be Sondheim’s first new musical since “Road Show,” which has been going through a difficult birthing process since 1999; it played at the Kennedy Center as “Bounce” in 2003, reopened as “Road Show” at the Public Theater in 2008, and had a production at the Signature Theater this spring.

Sondheim brought up the premiere in the context of a public discussion with Jamie Bernstein (Leonard Bernstein’s daughter) about the difference between musicals and operas. The composer Thomas Adès has just premiered an opera based on “The Exterminating Angel,” which has been warmly received by some critics at the Salzburg Festival; the work, Sondheim said, is coming to the Metropolitan Opera in 2017. Sondheim encouraged the Glimmerglass audience to attend both works.

“It will be an interesting comparison,” he said, noting that Adès’s opera, based on a single film, runs some two and a half hours (according to him), while the same film represents only half of his own upcoming work.

In the course of the discussion, he delivered a number of pithy and sometimes familiar observations about writing for the stage. “You don’t take a topic and write about it,” he said; setting out to write, for instance, a piece “about” homosexuality was in his opinion a recipe for failure. “The topic arises out of whatever story you tell,” he said. As for “Sweeney Todd’s” subject: He wrote it, he said, simply “to scare an audience.”

Peter Marks contributed to this report.