"Showing up is 80% of life." The quote's attributed to Woody Allen, but Fringe veteran Peter Michael Marino has made it his own – booking an hour-long slot at a premier free venue, and then, if you believe him, simply turning up to perform. It's not quite true of course; a lot of careful preparation's gone into this interesting solo show, but it's still a rousing testament to the freedom of spontaneity and a rallying-cry for Marino's second motto, "Don't think, do".

The opening segment is a perfectly serviceable stand-up routine, built round anecdotes from Marino's past life. He talks about the "misfortune" (for a comedian) of having a happy family, discusses the unreliability of memory, and gently skewers the Catholic Church. It's all reassuringly familiar material, but there's a hint of pain visible under the surface. And it proves to be the warm-up for a memorable second act – where Marino delivers on his "don't think" mantra by improvising a story, helped or hindered by a "stage manager" and "technician" he selects from his front row.

There's plenty of improvised work at the Fringe these days, but Show Up stands out because it isn't overtly comic. There's humour there of course, and Marino works in a good smattering of hilarious lines, but at heart he weaves a serious story – which on the day I attended, took a surprisingly dark yet somehow fitting turn. The narrative did wander from time to time, but there were also elegant call-backs, and a clever metaphor comparing NASCAR racing to passage through life.

The show is distinguished, as well, by the rapport Marino builds with his audience, and the genuinely supportive atmosphere he creates in the room. In place of the normal patterns of comic improv (shout out a location! a hilarious profession! a type of fruit!), Marino uses trigger words like "family" to gather stories from his audience – which he summarises onto Post-It notes, and respectfully works into his plot. On the day I attended there were plenty of people who wanted to share their secrets, and Marino successfully found the essence of both outrageous and more sober responses to feature in his improvised storyline.

So what's the point of it all? It's subtly done – but there's a darker tale lurking here, about what happens when the urge to show up deserts you. Marino's ultimate message is that even when we're not ready for what the world throws at us, the first step is just to carry on. It's an affirming thought, which Marino matches with an unexpected and touching conclusion. All in all then, this unusual performance is well worth showing up to.