“Theater is as big an interest in my life — interest meaning something I love — as movies or as music,” he said. “They’ve always been equal. And therefore it occurred to me: Why not combine music and theater? That’s called musical theater!” And to boot, he added, “I was brought up by Oscar.”

As Sondheim fans know well, he was referring to the lyricist Oscar Hammerstein II, who became not just his mentor but also a kind of surrogate father after his parents divorced and his mother moved with her only child from the Upper West Side of Manhattan to rural Pennsylvania.

Mr. Sondheim was 11 when he met Hammerstein and his family. For five years, this musical theater giant gave him invaluable insights into the art form. “Oscar — let’s not say he was a poet — was a lyricist and a playwright,” Mr. Sondheim said. “Why wasn’t he a novelist? Because he loved the theater!” That’s the “simple answer,” Mr. Sondheim said, to the question I raised.

Mr. Sondheim’s musical training, such as it was, started early.

“When I was 7 years old, like all nice Jewish boys on the Upper West Side, I took piano lessons,” he said. “When my parents had company over for cocktails, I would be trotted out to play ‘Flight of the Bumblebee’ because I had a very good right hand.” (His left hand, he asserted, was “a lox.”)