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When thinking of Kelli O'Hara, the five-time Tony Award nominee who has starred in revivals of classic musicals like South Pacific and The Pajama Game, the words "drunken brawl" don't often come to mind. But that's just what O'Hara will be doing when she takes the stage at the Metropolitan Opera New Year's Eve in The Merry Widow.

Directed by Susan Stroman (The Producers, Bullets Over Broadway), The Merry Widow features O'Hara alongside Renee Fleming in Lehár's operetta about a rich woman and her countrymen's attempt to keep her money in the principality by finding her the right husband. O'Hara, who has also starred in The Bridges of Madison County, Nice Work If You Can Get It and Sweet Smell of Success, called working on an opera with Stroman, known for her lively choreography in musical theatre productions, "the best of both worlds."

"I'm doing the whole first part of the show, which is more opera, and then I get to finish the show with this drunken brawl kind of thing," O'Hara said, laughing.

Prior to making her debut at the Metropolitan Opera, a life-long goal that she described as leaving her "shaking in my boots a little bit," O'Hara will take the stage at Carnegie Hall with Matthew Morrison in Kelli and Matthew: Home for the Holidays Dec. 19-20. Accompanied by the New York Pops, the duo will perform seasonal holiday classics as well as some old favorites. Morrison (Hairspray, Footloose) and O'Hara, who co-starred in The Light in the Piazza and South Pacific, have wanted to present a concert together for many years. Performing in a holiday concert marks a return for O'Hara to a tradition that began at Oklahoma City University, where she studied vocal performance and opera and would perform in an annual holiday concert.

"The years that I didn't perform in one of those made me feel like I was missing something," O'Hara remembered, "so to do a big, huge holiday concert with a huge orchestra like this kind of takes me back and makes me feel like I'm fulfilling my tradition as well."

Holiday music marks a tradition for Morrison as well, who released the holiday CD "A Classic Christmas" in 2013, and has heard his own voice in various locations throughout the holiday season.

"I heard it at a Jamba Juice the other day. I heard it at LAX, the airport," he said. "I get a little embarrassed. I kind of put my head down - this is a little awkward. But it's a fun little treat as well. It brings you back to the time and all the effort and the people I got to work with. [It makes me] a little nostalgic. "

The lineup will include some classic holiday numbers like "Oh, Holy Night" as well as the family-friendly "Believe" from the animated film "The Polar Express." O'Hara said she loves being a part of someone else's holiday celebration, adding, "I think the holidays are such traditional things for families. It was really hard in the beginning of my career to start stepping away from that or not having it. I would always feel kind of empty, like I wasn't having an actual holiday.

"But the best thing about doing something professional for the holidays is that you look out there and you realize you've become part of another person's tradition," she added. "That was interesting for me to understand: That we do a show on Christmas Day and that's actually a tradition for some families."

While performing during the holidays, the casts and crews of shows create their own traditions, O'Hara said, including Secret Santas, white elephant gift swaps and even a pajama party during the run of The Pajama Game. ("It was just the most cozy, comfortable party you've ever been to in your life," she said.) Morrison will be bidding goodbye to his own traditions when the final season of "Glee," the hit FOX musical series that airs the first episode of its final season Jan. 9, 2015. Morrison, who stars as the high school choir director Mr. Schuester, described the experience as "bittersweet," saying, "We've gone through a lot of really great highs and terrible lows. But for the most part it's been such a great journey. I can't believe we've been doing it for almost seven years now."

The success of "Glee" was a surprise, Morrison said. "It's so funny because I think about that first episode airing. Before it even started, we didn't know what the show was going to be or if it would be successful. I kind of thought it was a niche kind of show that some people would like but never expected it to be what it became."

After reuniting at Carnegie Hall, O'Hara and Morrison will both return to Broadway in 2015, with O'Hara starring in The King and I as Anna Leonowens and Morrison playing J.M. Barrie in the new musical Finding Neverland.

"I feel a little added pressure – my return to Broadway – but I am so excited for the challenge and so excited to tell this amazing story in this show that I really fell in love with while doing the workshop on it last March," Morrison said. "I was really hoping it would come back around and I'd be able to do it on Broadway. It's amazing. It's telling the story behind the story, which is the part that I find the most interesting and fascinating about how J.M. Barrie came up with this crazy place called Neverland."

With busy schedules waiting for them in 2015, O'Hara and Morrison said they're looking forward to taking the stage at Carnegie Hall and being a part of families' holiday celebrations.

"This is New York. This is the kind of place where traditions are a little bit different," O'Hara said. "It's so celebratory because you realize you're enhancing their Christmas or holiday. It's more of a different kind of tradition I think, but you're part of it."'

(Carey Purcell is the Features Editor of Playbill.com. Her work appears in the news, feature and video sections of Playbill.com as well as in the pages of Playbill magazine. Follow her on Twitter @PlaybillCarey.)