As for moving between the worlds of opera and showbiz, Gunn was trained from the start. "My first teacher, who was 84 when I stared learning from him, had a career in opera and oratorio. But his main job was being top tenor in a quartet on the Carnation Breakfast Radio Show. He was used to adapting his vocal approach and that's how I was taught."
Gunn seems a little defensive when I bring up his adulation by the admirers of so-called "barihunks". "It's taken six or seven years to shed that image," he sighs, pointing out how it all started with a 1998 production of Gluck's Iphigenie en Aulide.
"Francesa Zambello, the director, told the tenor William Burden and me that we were going to come in stripped, covered in water and chained together - if you have a problem with being half-naked on stage, she warned, then start going to the gym."
Bodily exposure was never a worry for the athletic baritone, but he chuckles as he says that "they sold out of opera glasses for that production - and this was not a big theatre".
He met Patinkin when they were sharing a dressing room for Stephen Sondheim's 80th birthday concert in 2010. Gunn recreates the moment. "I just love your voice," he says, playing Mr P. "I don't know if my ego can stand being on stage with you but I think we should try to do something together." Gunn took up Patinkin's suggestion and the resulting set of songs and banter, accompanied by the two pianos of Paul Ford and Julie Gunn, was "planned like coming to someone's house for dinner".
"It's never the same from night to night," he points out. "The main dishes are there, but the talk around the dinner table can change. It becomes a living, breathing conversation that everyone in the audience gets to be part of."
Those who recall Patinkin having flirtatious fun with Patti LuPone four years ago will not be getting a repeat performance.
"It's hard doing a show with another man," Gunn admits. "But I think we have a special dynamic. We respect each other deeply and there are times when the one just sits back and appreciates what the other one is doing - with the same enjoyment that we hope the audience will be having out in the theatre."
Performance
What: An Evening with Mandy Patinkin and Nathan Gunn
Where and when: Aotea Centre, November 24 at 6pm