Heraclitus famously opined that you can never step in the same river twice. If he spent more time attending theater than stepping into rivers, he may have said you can never attend the same improv show twice.

On the Spot, an improvisational performance at the Broadway Comedy Club, produced by Nathan Armstrong and directed by Pat Reidy, presents a show where every experience is necessarily different. The performances consist of singers and improvisational performers (Players) who are unaware in advance of the night’s lineup of songs and turn each song into an improv act. Although the singers and Players change from performance to performance, the format is consistent. On the Monday evening that I attended, five singers (Nora Kennedy, Claire Elise Walton, Mason Reich, Mal Walton, and Audrey Hughes) each performed two songs. During each song, the singer was flanked stage right by a gifted pianist (Andrew Whitbeck) and stage left by four improvisational Players (Patrick Reidy, Chris Catalano, Meg Reilly, and Andrew Del Vecchio). The actors quietly waited and as soon as the song ended, they jumped into action turning each song into a humorous five-minute routine.

The song choices were an eclectic mix of oldies, parodies, obscure musical theater, and a sprinkling of Adele. The singing was fine but there were no breakout moments. If I were to be especially critical, I would summon my inner Randy from American Idol and say the singing tended to be “a little bit pitchy.” But to be overly critical of the singing at an improv performance is a bit like going to the Met and complaining about the muffins in the cafeteria.

The magic of the evening began when the four improvisational actors would take a thread from each of the songs and weave it into an entirely new creation. The improvisers were clearly veterans of their craft, quick on their feet, silly, but also reactive and responsive to each other. A few examples from the evening demonstrate the variety of ways that the Players crafted a skit:

Mal Walton sang Accident Prone by Laurence O’Keefe and Kirsten Guenther, a song told from the perspective of a klutzy woman describing a series of ex-boyfriends — some whom she inadvertently injured. She was unlucky in love and unwilling to settle in her quest for anything less than a “chill” boyfriend. The Players interpreted this song literally, acting out a support group for all the ex-boyfriends. Chris Catalano played a memorable ex-boyfriend who had his tongue accidently bitten and who was learning to adapt to speaking with a lisp and living without enjoying ice cream.

Audrey Hughes sang Skyfall by Adele, notably featured in the eponymous James Bond film. The Players of course played off the James Bond angle, but added a twist to the secret agent trope. Andrew Del Vecchio emerged as the novice secret agent 003, lacking the ruthless skills of Bond, appearing confused and somewhat apologetic about MI6’s odd numbering system for its agents. While trying to foil the evil villain’s dastardly plan, he mistakenly unplugged the cord to a toaster rather than the death ray threatening mankind.

Claire Elise Walton, the strongest singer of the night, sang Everybody Says Don’t, written by Stephen Sondheim (which included the lyrics “Everybody says can’t fight city-hall/ Can’t upset the court/ Can’t laugh at the king!/ Well I say try . . .”). The Players took an absurdist approach to this song, acting out an intervention for Jesus. Jesus’ friends criticized him for his stringent rulemaking and his frequent alcoholism (overdoing the turning water into wine). They were especially confounded by the fact Jesus frequently spent time with a prostitute but admonished his friends any time they wanted dessert because gluttony is a sin.

While a few skits dragged, most scenes were creative and varied juxtapositions to the songs performed. Improv was at its best when the premises opened up multiple humorous possibilities and I found myself wishing many of the 10+ skits went on longer. But the quick pace ultimately worked to the show’s benefit because when the chemistry and direction of the performance worked well, the audience was left wanting more. And in skits where a premise hit a dead end, the bell could save it.

One of the most fun experiences as an audience member was seeing an actor’s range as he created different roles on the spot. Chris Catalano was equally comfortable playing a swagger-filled and enthusiastic game show host as he was playing a sad, self-reflective ex-boyfriend sans tongue (and with a speech impediment).

Throughout the evening the Players also developed a certain rapport with the audience, invoking inside jokes developed through the skits. Patrick Reidy found a shockingly high number of ways to incorporate the lead singer of Alien Ant Farm into skits – much to the delight of the audience. Meg Reilly insisted she could speak parseltongue, the serpent language from Harry Potter, a promise that disappointingly never came to fruition.

With improv, some of the fun is watching the gears turn in the minds of the Players, and watching their instantaneous reactions to whatever their fellow Players throw at them. And sometimes parseltongue never gets a chance to be spoken. But it’s clear that whatever premise the Players are given, they can transform it into an enjoyable evening.

On the Spot runs Monday nights at the Broadway Comedy Club at 318 W. 53rd St.