"My view is that Messiah has remained popular because it tells the greatest story ever told," he says. "It's part of people's religious life, it tells a story of Christian redemption from start to finish."
Stevenson talks fondly of past performances and says singing with professional soloists helps keep things exciting for the members of the choir, who are mostly part-timers. Tenor Simon O'Neill is the big star of this Messiah, though Handel is not a composer normally associated with the singer, who, these days, is more likely to be heard in weightier music.
"Simon started in youth choir," Grodd says. "He would have grown up singing Messiah. His voice has the capacity to fill the Town Hall but he's at an age where that voice is at its prime, so he can do anything with it. I'm really looking forward to that first [aria] Comfort Ye, when he opens his mouth and sings."
O'Neill is the standout name but Auckland Choral is doing a couple of other special things to mark 100 years.
Most notably, this will be a big Messiah. Early performances conducted by Handel in the 1740s and 50s featured around 20 choristers and a similar number of orchestral musicians. Grodd's orchestra, Pipers Sinfonia, will boast 50 players, which is large for a Baroque band, but he's really gone to town with the choir, cramming almost 250 singers into Auckland Town Hall.
There is precedent for such large forces; Grodd says that even in Handel's lifetime, Messiah grew bigger and bigger. By the middle of the 19th century, things had got entirely out of hand. In 1859, a performance to mark the centenary of Handel's death comprised 460 players and 2765 singers.
Auckland Choral is going nowhere near that but Grodd has nonetheless needed to supplement his usual group with members of the University of Auckland Chamber Choir, Dunedin Civic Choir and Brisbane Chorale. In a nice touch, he's also invited alumni of Auckland Choral to return, on the understanding that they commit to seven rehearsals and a weekend workshop ahead of the concert.
Such extended rehearsal time is unusual. Soprano Anna Kofoed joined Auckland Choral as soon as she arrived from England in 1984 but her association with Messiah goes back to the 1960s, when she was a schoolgirl in Oxford. She remembers that when she sang in the Royal Choral Society in London, Messiah might only earn one or two rehearsals, if you were lucky.
"They did it every year and didn't think you needed a lot of rehearsals. If you'd never sung it before you were at a huge disadvantage."
Grodd, you sense, would not leave his singers at a disadvantage. For him, it's less about perfection than the search for transcendent moments or what his teacher, the great Sergiu Celibidache, called the "yes".
"Celibidache, used to ask: how many yesses are there in music and how many nos?" explains Grodd. "There are lots of nos: too flat, too sharp, too early, too late, too loud, too soft. Everything is no, no, no, no, no. And then there comes a yes. The yes is that moment when everything clicks, when everything is one." In Handel's words, it's the Hallelujah.
Lowdown
What: Auckland Choral, Handel Messiah: 100 Hallelujahs
Where and When: Auckland Town Hall, December 16 and 17, aucklandchoral.com
