When Anthony Roth Costanzo opens his mouth onstage, audiences usually are a little surprised, and that’s how Costanzo likes it.

“It's a kind of shocking thing for people who've never heard one before, and I love that, I love the novelty of it,” said Costanzo, 34, referring to his countertenor voice. The slim but physically fit Costanzo, who started performing as a kid with his sights set on Broadway, has made a name for himself singing in a high register.

After he performed during last year’s Spoleto Festival in the Bank of America Chamber Music series, talk of Costanzo’s return helped organizers decide to mount the U.S. premiere of Vivaldi’s “Farnace” — an 18th-century opera that’s rarely performed and requires countertenors in the cast. The opera runs at the Dock Street Theatre; it's conducted by David Bates and directed by Garry Hynes.

Hynes, the respected director of the Druid Theatre, based in Galway, Ireland, made her operatic debut at the 2014 Spoleto Festival with Leos Janacek’s contemporary Czech opera "Kat’a Kabanova.” This year, she's especially busy at the festival, also in charge of the Druid's production of "Waiting for Godot," also at the Dock Street Theatre.

“Farnace” will be “another world with its own challenges,” Hynes said, noting that it was written before the advent of the more familiar bel canto opera style. Hynes has been looking forward to the challenge and to working with Costanzo.

“I've been absolutely fascinated by the countertenor voice, and I think it's one of the most dramatic things in drama,” Hynes said. “But I've never actually worked with a countertenor, so Anthony's going to be my first.”

The two met this past winter in Los Angeles when Costanzo took in a performance of “The Beauty Queen of Leenane,” which Hynes, who won a Tony Award for it in 1998, was revisiting as a director.

Hynes familiarized herself with “Farnace” by watching video clips of other performances. Based on Pharnaces of Pontus, who lived between 97 and 47 B.C., the operatic hero Farnace is a king who, having been defeated by the Romans, orders his wife to kill her son and then herself to save them from being captured and losing their honor.

Talented countertenors like Costanzo are one of the reasons more baroque operas like “Farnace” are now being staged, said Wendy Heller, a professor of music with a background in Italian baroque opera at Princeton University, Costanzo’s alma mater. While wonderfully dramatic, baroque operas are often considered less accessible, she said.

“Listeners accustomed to Handel’s operas will certainly find Vivaldi’s style — particularly at this latter part of his career — somewhat different,” Heller said. “There is an intensity to his writing for both voice and instruments that is extraordinarily dramatic, and he is particularly good at writing virtuosic music for his main characters.”

Heller, who once taught Costanzo and now calls him a friend, saw him perform in “The Enchanted Island” at the Metropolitan Opera in 2012. As the understudy for acclaimed countertenor David Daniels, Costanzo made an impression.

“He just owned that role, he had it,” Heller recalled. “He's one of the few people I've ever heard who has the courage to sing really softly on the Metropolitan Opera stage. Everyone just leaned in to hear.”

But Costanzo also can belt, and the range required in “Farnace” is extensive.

“Vivaldi loved to write for the violin — and we all know his famous ‘Four Seasons’ — but he writes for the voice the exact same way that he does for the violin,” Costanzo said, noting his arias jump up and down octaves. “There's a lot of fast notes flying everywhere.”

Also flying everywhere is Costanzo himself. Before coming to Spoleto, he was performing in “Only the Sound Remains” by Kaija Saariaho at the Finnish National Opera. Another recent performance had him tackling the title role in Philip Glass' “Akhnaten” with the English National Opera.

An expert at keeping himself busy, Constanzo will juggle his role in “Farnace” with performances in Spoleto’s chamber music series. Chamber music director Geoff Nuttall said he is as excited to continue working with Costanzo as he is to see “Farnace.”

“It's an incredible voice, an amazing dramatic presence and vast repertoire,” said Nuttall, who met Costanzo last year when the countertenor entertained chamber music listeners with both Gershwin’s “Embraceable You” and works by Handel. “We've been so lucky.”

During this year’s chamber music series, Costanzo will perform Vivaldi arias, as well as a new arrangement of Roy Orbison’s “Crying,” which Orbison’s widow previously asked Costanzo to sing.

“I do think I have a unique sound, and I don't know exactly how to describe it, other than to say that, for me, the sound comes from the emotion,” Costanzo said. “I'm less interested in constant beauty or absolute perfection. What I'm most interested in is moving people.”

Dara McBride is a Goldring Arts Journalist at Syracuse University.