What turns a magical moment onstage into a true showstopper?

You can't stop a showstopper.

What it comes down to is an agreement between the folks on stage who do the thing and the folks in the audience who decide the thing is so spectacular that hands must be clapped and attention paid.

It is the audience that forces actors to hold an extra few seconds after iconic numbers such as the tap-dancing chorus of "Anything Goes," or "Rose's Turn," the breakdown song in "Gypsy" that Michelle Barber nailed in Theater Latté Da's 2016 production, or gangster Nicely-Nicely Johnson's "Sit Down You're Rockin' the Boat," the revival-meeting "Guys and Dolls" tune some lucky actor will tackle next summer at the Guthrie.

Each of those has the potential to give you goose bumps, to produce a halo of feeling that makes you think, "Holy cow, I'm watching something special here."

But a stellar song won't always stop the show.

"If you don't have a great Nicely-Nicely, you may not get the showstopper. But Mandy Patinkin could probably turn even a mediocre song into a showstopper," says Michael Brindisi, who has seen many a show stopped in 30 years at Chanhassen Dinner Theatres and is currently planning a showstopper, featuring tap dancing and jumping rope, for this fall's "Holiday Inn."

Sometimes, a song is built specifically to whip the audience into a frenzy: "Friend Like Me," from last year's touring "Aladdin," packs in tap dancing, a chorus line, impressions, magic tricks and special effects. "Something Rotten," which toured to the Twin Cities earlier this year, boasts a legit showstopper ("A Musical") but nothing else. That's why Brindisi thinks showstoppers can be a bad thing, with some productions and performers so focused on their big helicopter/chandelier/high C that they ignore everything else a show needs.

Generally, a classic showstopper comes near the end of the first act (think "And I Am Telling You I'm Not Going" in "Dreamgirls") or the finale ("The Winner Takes It All" in the Ordway's "Mamma Mia" this summer), whipping the audience into a fever before they head to the restroom or home.

But rules are made to be broken. When the Guthrie did "The Music Man" in 2015, the opening number, "Rock Island," was performed with such verve it stopped a show that had barely started.

A bit of physical comedy can stop a non-musical, too, like the puppet sex scene that left audiences gasping for breath this summer at Jungle Theater's "Hand to God."

Even a costume can do it, especially if Sally Wingert wears it.

"Anything that person puts on — unless she's playing a character who has to blend in, which is not often — becomes a showstopper," says Guthrie artisan Douglas Stetz of Wingert, whom he has costumed for 27 years.

"You don't build costumes to stop shows," says Stetz, who describes his all-time favorite costume, a brown poncho worn in the original "Dreamgirls," and the "Blithe" cape in similar ways: In both cases, the drama comes from an actor unfastening a button that makes her outer layer of clothes cascade to the floor, revealing the magnificence beneath.

"We always knew there was going to be a cape. We didn't know what she was going to do with it: Walk out with it on her arm? Hand it to a servant?" says Stetz. "I would lay odds that Sally, at some point in rehearsal, said, 'I'll just drop it when I walk in.' I'm sure Sally was in complete control of how much time she gave the audience to react, and they loved it. She told me that cape had a fan club."

Unlike "Rose's Turn" or the volcanic "And I Am Telling You," that costume was a quiet showstopper, an almost casual effect that hit the audience just right. Ben Bakken is hoping for something similar with "Falling Slowly," in "Once," which opened this weekend at Theater Latté Da. A gentle duet, it comes relatively early in the show and, in keeping with the subtlety of the musical, underscores an ambiguous encounter between the two main characters. But "Falling Slowly" also is an Oscar winner, a radio smash and the one thing in "Once" that just about everyone in the audience expects the performer to ace.

No pressure, Ben Bakken.

"I think what makes it a showstopper is it's the first time the [unnamed] Guy and Girl connect on a spiritual level," says Bakken, who sings it with Britta Ollmann. "She's learning the song from the sheet music and he obviously already knows it, but he does not know her. So playing the song together is this special moment. It doesn't really propel the story. It's more a moment when everything stops. It's very simple. I hope I can do it well."