Most Broadway musicals that close after 16 performances barely prompt memories, let alone documentaries. But in 1981, the Stephen Sondheim/George Furth opus, Merrily We Roll Along, rolled along so bizarrely, it became the stuff of Broadway legend, worthy of a 2017 post-mortem. Best Worst Thing That Ever Could Have Happenedis a theatrically captivating documentary in which a director looks sideways at a musical that goes backwards.

In the opening moments of the film, director Lonny Price is rummaging around in a cardboard box filled with film from an ABC-TV special about Merrily. The special never aired — it was scrapped when the show closed so quickly — and he's startled to find 35-year-old footage of his younger self. Barely out of his teens, Price was cast as one of the show's leads, along with a couple of dozen other 16-to-25-year-olds. Now as a first-time film director, he's looking back at his own professional acting debut.

Once he cues up the filmed interview, the kid staring back at him from the screen is a dewy-eyed, frizzy-haired innocent, thrilled to be cast in the first project director Hal Prince and composer/lyricist Sondheim tackled after their Broadway triumph with Sweeney Todd.

"I walk around smiling all day," he says. "This show, if I never do anything again in the rest of my life, I will have had this moment. If I get hit by a truck the night after the opening, I don't think I'll care."

He was perfect casting for the exuberant youngster he was playing. You can hear it in his voice on the cast album, as he sings: "It's our time, coming through, me and you man, me and you."

This song, though, isn't how the show starts; it's how it ends.Merrily We Roll Along is about college pals whose friendship sours over time, but as Price explains in voiceover, it's told in a way that sweetens over time: "The big conceit of the show is that it goes backwards," he says. "These unhappy characters start in their 40s, and in each following scene, it's a few years earlier; they're a few years younger, a few years less bitter, less jaded, until finally at the end of the show they're ... optimistic and full of dreams, with no idea of what's to come."

You could say that about the show's creators, too. Prince and Sondheim were pretty young back then, and a string of Broadway hits — Company, Follies, A Little Night Music, Sweeney Todd — had not prepared them for what was about to happen with Merrily. Oh, it started out like a song — nothing but excitement in rehearsals, these gods of Broadway working with kids who idolized them.

One youngster, Jason Alexander, remembers what it was like the first time there was an audience out front: "I don't think anything will ever top being behind the curtain just before the overture started at the first preview," he recalls.

(And that's saying something — this is the guy who later played George on Seinfeld, after all.)

But halfway through the first act, it all started coming apart. This going backwards thing, and kids playing adults ... the audience didn't get it. One cast member remembers whole rows getting up and leaving. Another remembers singing to the backs of people walking out, which she terms "not a subtle cue" that the show had problems.

This first part of Best Worst Thing will be absolute catnip for Sondheim fans — the ecstasy and the agony, as it were. And then, in the documentary's second half, director Price does something unexpected. You think he'll chronicle what happened to the show — which is basically that after it flopped, the creators figured out how to fix it so that it gets produced all the time now. Instead, he does what Merrily does: He concentrates on what everything from disappointment to wild success did to the people involved.

Their trajectories are riveting, because Best Worst Thing That Ever Could Have Happened has all this footage of them when they were starting out — including that interview with the young Lonny Price, that the grown-up Lonny Price was watching at the beginning of the film.

He plays it again towards the end, and this time, you watch him watching. Couldn't feel more different.

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