"A Little Night Music," the award-winning Stephen Sondheim musical premiering tonight at Blaisdell Concert Hall as a Hawaii Opera Theatre summer attraction, arrives with challenges for its cast.

It's a Broadway hit that, in addition to operatic singing, requires acting finesse. And it's written in three-quarter waltz tempo — a device that's not all that daunting but surely different. Its vocal style is more sustained and mood-evoking, and less influenced by show-stopping arias.

The production deals with regrets and infidelities involving a middle-aged widower and his young bride. The musical's romantic entanglements are inspired by the Ingmar Bergman set-in-Sweden film, "Smiles of a Summer Night."

For this assembled cast, "Night Music" is a chance to adapt and explore.

"It's the first time I am amplified with a body mike," said Jake Gardner, a seasoned bass-baritone who portrays Fredrick Egerman, a lawyer married to an 18-year-old. Their 11-month marriage has not yet been consummated.

"I've been singing (in operas) without a microphone. So, I've had to regulate my singing and my speaking voice," Gardner said. Easy enough, he said. For Gardner, some of the musical's challenges start when the singing stops.

"This is a great piece; the acting is part of the reason I was attracted to 'Night Music,' " he said. "I would even like to do a straight play, with no singing, as the ultimate challenge someday."

Diane Alexander, playing Desiree Armfeldt, a somewhat self-absorbed actress who regrets not marrying Fredrick 20 years earlier, is tackling her first Sondheim. And she's the one who delivers the poignant key tune, "Send in the Clowns."

"This is kind of a breed in itself," said Alexander of Sondheim's musical theater style.

"I've done a lot of operettas before. So, I'm used to music and dialogue," she said. Regarding "Send in the Clowns," she added, "This is such an iconic song, which can be either intimidating or completely satisfying. In the text of the show, you need to know what the song is about. And when you see it in the context of the show, it's amazing."

To become Desiree, Alexander said, she had to turn on her warm faucet, to bring warmth and humanity to the part.

"Diane Alexander is not a cold, complicated person; I have to say an amazing line to Rosalind Elias, who plays my mother, Madame Armfeldt, 'You are in one of your bitchy moods, I see.' I have a hard time saying that to her; I don't call anyone bitchy," she said.

"You have to separate who you are from the character, but I love the fact that I can be cold and I can play warm. Desiree's spirit and zest for everything in life come through the character; she's nearing the end of her career as an actress and it's fun to play an actor in the show."

For Gardner, this is not the worst way to spend a summer — in Hawai'i, on a semi-vacation mode.

"For a four-week visit, the music is not vocally demanding, and there's a little more beach possibilities," he said. Further, his wife, Jill Gardner, is aboard the show, too, playing Countess Charlotte Malcolm.

Alexander said because Desiree does not do a lot of singing, she has an added challenge "to keep my voice warmed up. I warm up early on, before the show; by intermission, I feel I need to warm up again," she said.

She's also wrestling with dialogue and has reached a comfort zone with director Henry Akina's input regarding her speaking voice.

Some years ago, Alexander delved into musical theater by portraying Carlotta, the opera diva in "The Phantom of the Opera,"at the Curran Theatre in San Francisco. It was pop opera, entirely sung, with words moving the story.

"That was brutal — a totally different animal, being on stage every day, doing eight performances a week," she said. "But I learned so much, about the stage and the craft. I came away exhausted but happy."

Gardner said while opera — still his bread and butter — is attracting a younger breed of fans, it's a survival tactic for singers like himself to foray into musical theater.

"I'd have to get a Broadway agent, and have to get used to being tied down for a period of time, but there are parts I'd like to do, like Don Quixote in 'Man of La Mancha' and (Emile) de Becque in 'South Pacific.' "