NEW YORK >> Marin Mazzie found herself last Friday wearing a lilac ball gown hoop skirt and a determined expression.

She was waiting to go onstage for the top of Act 2 of the lavish Broadway production of “The King and I” at Lincoln Center. This was her first and only rehearsal of the show in full costume with all the other actors and the orchestra before she does it for real Tuesday night in front of an audience.

“Ready to give it a try?” asked Lisa Chernoff, the assistant stage manager.

“Let’s do it,” replied Mazzie, before striding out to play the leading part of school teacher Anna Leonowens.

Leaping aboard a finely-tuned musical like “The King and I” isn’t for the faint of heart, but on this day Mazzie, thankfully, had company: Her King of Siam was Daniel Dae Kim and it was his first full show, too.

Replacing leads in long-running Broadway shows isn’t unusual, but getting two leads ready for their first time in a show on at the same night is rare. Talk about whistling a happy tune.

To add to the stress, Mazzie is replacing Kelli O’Hara, who won a Tony Award in the role, while Kim, who stars on the TV series “Hawaii Five-0,” is making his Broadway debut.

“I can see where there’s work to be done. At the same time, I’m not too hard on myself because there are a lot of new elements that we’ve never rehearsed with before,” said Kim, who played the same monarch in London in 2009.

For Mazzie, her stage debut in “King and I” comes virtually a year after she was diagnosed with ovarian cancer. The veteran of such Broadway shows as “Ragtime” and “Passion,” last April was ill and singing the lyric “Life is what you do while you’re waiting to die” in the musical “Zorba.” Now she’s signing “Shall We Dance.”

“It’s very emotional for me,” she said. “I’m so anxious and excited and thrilled to be able to bring, in essence, a new me back to the stage with what’s gone on in my life.”

Signs of nerves were hardly evident during the rehearsal in front of a handful of people at the massive Vivian Beaumont Theatre. The mood was light and cast members crept into the massive hall to cheer and clap the newcomers when not needed onstage.

The rehearsal had a slightly surreal quality since only the show’s new additions — Kim, Mazzie and two of the 11 Thai children — were in full costume. All the rest wore sweats, T-shirts, hoodies and Converse.

Director Bartlett Sher stopped the action only twice, once to ask Kim to project more on the tricky song “A Puzzlement” and another time to ask that everyone pick up the pace in a pivotal scene in Act 2.

The musical’s story centers on an Englishwoman who travels to Siam in the 1860s to teach the children of the king. The 51-cast-member revival won the Tony Award in 2015 starring O’Hara and Ken Watanabe. Its score by Rodgers and Hammerstein includes “I Whistle a Happy Tune,” ”Getting to Know You” and “Shall We Dance.”

Ruthie Ann Miles, who won a Tony playing Lady Thiang, complemented Kim for having great instincts. “He’s really digging into the legacy of the king,” she said. “That’s what keeps it fresh.”

As for Mazzie, Miles is just happy to work alongside her: “It’s really fun to watch someone who I’ve admire for so long step into a role — a moving train — and go with it. It’s inspiring. It’s not easy what they’re doing.”

To get ready, Mazzie and Kim spent hours rehearsing together in a basement room with Sher and the stage manager, who would play five or six different roles.

“Having someone like Marin come in with me is a real blessing because we’re finding this show together,” said Kim. “The fact that both of us are working together and finding our King and our Anna makes it a really, really fantastic situation.”

Kim, who graduated from New York University with a master’s degree in acting and cut his teeth in off-Broadway shows, said he often would come to Lincoln Center.

“I’ve seen so many shows here,” he said. “I would wish that someday maybe I would be on that stage. Someday is now.”