In the early ’90s, at several memorial services for friends who had died of AIDS, I played “Good Thing Going,” a wistful song about recalling imperfect but cherished relationships. “Marry Me a Little,” cut from the original production of “Company” but beloved in later revivals as the protagonist’s statement of determination and despair, was another piece I relished performing; I still use the demanding perpetual-motion piano part as an exercise to keep my finger technique limber.

In 2010, I made an 80th birthday tribute video to him for the Times website, in which, among other excerpts, I played and analyzed the wondrous chords at the start of “Sunday in the Park with George.” Here, the hero, Georges Seurat, speaking to the audience, explains the elements of painting, how the artist must bring “order to the whole” through design, composition, balance, light — and, finally, harmony. Each word is accompanied, almost musically illustrated, by a variant of a five-note arpeggio figure that uncannily embodies each concept. The chord for light is so piercing and bright you almost want to squint.

In 2016, I posed to Sondheim the question of why such a master composer so seldom wrote a purely instrumental work. Yes, he was one of the greatest lyricists in the history of musical theater. But wasn’t he tempted to put words aside now and then, and just compose, say, a piano sonata?

He answered that it wasn’t really the words that generated his musical ideas. “I express the character,” he said. “Let’s see what happens to him. I express it musically.” He was endlessly fascinated by the “puzzle of music,” he added. But when he gets solely into music, the “puzzle takes over.”

I’ve been thinking since his death about a trip to the Bronx Zoo my husband and I took in the spring of 2019 with Sondheim and his husband, Jeff Romley. They were passionate animal lovers, and my cousin Kathleen LaMattina works there with her husband, Jim Breheny, the zoo’s director. In a special room, these honored guests could pet sloths and penguins, and even get close to a cheetah, under a staff member’s calm control. I have pictures of Sondheim feeding leafy tree branches to a giraffe.

I’m looking as I write this at the piano-vocal score to “Sweeney Todd” Sondheim signed for me the first time he came to dinner, in 1997.