In one afternoon, composer and lyricist Stephen Sondheim learned nearly everything he knows about songwriting.

As the story goes, Sondheim’s childhood friend was the son of Oscar Hammerstein II, the lyricist and playwright responsible for creating such masterworks as “The Sound of Music,” “The King and I” and “South Pacific” with composer Richard Rodgers. As a teen, Sondheim asked Hammerstein to review a musical he had written as though he was an adult.

Hammerstein’s verdict? He said it was the worst thing he’d ever seen.

He then explained to a crestfallen Sondheim everything that was wrong with the teen’s work. That moment, and the mentorship that followed, would prove formative for Sondheim, not-so-arguably the greatest innovator of the American musical in the past half-century.

From his lessons with Hammerstein through all that followed, Richmond Triangle Players’ new show aims to explore Sondheim — creator of “Company,” “Sweeney Todd,” “Sunday in the Park with George,” “Into the Woods” and others — in his own words. “Sondheim on Sondheim,” a musical revue of Sondheim’s craft, intersperses his songs with filmed interviews of the composer himself.

“It’s not a retrospective,” explains director Doug Schneider. “It’s a look at a lot of his material, some of it familiar, some of it never heard before.”

With more than 30 songs, “Sondheim on Sondheim” shows how many of his works progressed, including three proposed closing numbers for “Company” and three conceived opening numbers for “A Funny Thing Happened on the Way to the Forum.” It even includes a tune from “By George,” the comic musical Hammerstein ripped apart from Sondheim’s salad days.

“You learn a lot about him, and you learn a lot about how he works,” Schneider says, including Sondheim’s habit of working while lying down, often with a drink and always with a soft-leaded pencil. “They tied a lot of his songs to these interviews, so in a weird way they comment on what he was saying.”

Local actor Dan Cimo, who sings “Franklin Shepard Inc.” from “Merrily We Roll Along,” says the selection of songs should surprise even the most ardent Sondheim lover.

“You’re likely to hear songs that you haven’t heard before,” Cimo says. “The show has so much heart to it, and so much of a relatability factor. You’re likely to see your life in some way onstage.”

In Triangle’s show, actress Susan Sanford sings “Smile, Girls,” a number written for Mama Rose in “Gypsy” that was ultimately dropped from the musical.

“It’s really for musical theater lovers, for people who enjoy hearing the back story of Sondheim himself, and also the material that he’s famous for,” Sanford says, adding that Triangle is the perfect venue for the show. “It’s a very intimate, cabaret style of musical, so I think it’s going to be very rich in there.”

Rachel Rose Gilmour, who has a duet with Sanford and performs the show-stopping “Ah, But Underneath” — a song cut from “Follies” — says that though he’s beloved as a composer, Sondheim isn’t the easiest to perform.

“He writes very complex music,” Gilmour says. “As a singer, it’s very difficult, because you’re not singing a simple ballad. His notes are constantly jumping all over the place.”

In addition to illuminating parts of Sondheim’s process, Gilmour says the show reveals the composer’s personal side through the interviews.

“He says something about his family life, about his mother specifically, and their relationship to one another, and it’s truly heartbreaking,” Gilmour says. “You can tell he doesn’t want pity. He’s just very honest and very real, and talks about how that shaped him as a composer.”

Performing “Being Alive” from “Company,” among others, actor Durron Tyre says Sondheim’s exploration of the human experience helps set him apart from other composers.

“I really enjoy singing his music, because it has so much heart and so much meat to it,” Tyre says. “His music is all based on [personal] experience, it’s all based on love and loss. It’s based on growing up.”

In summing up Sondheim’s importance, Schneider calls him a “genius of musical theater.”

“He was the one to take the old format, [like the work of musical writing team] Lerner and Loewe, and turn it on its head,” Schneider says. “He changed the way we look at musical theater.” S

“Sondheim on Sondheim” plays Aug. 8 through Sept. 1 at Richmond Triangle Players, 1300 Altamont Ave. For information, visit rtriangle.org or call 346-8113.