Playing a "Waitress" on Broadway isn't just another role for Katharine, who is known as 'Kat' to her friends.
It's her Broadway debut and a fulfillment of a long-held dream - a dream deferred since high school.
"It does mean more to me because I always knew I wanted to do Broadway, and quite frankly, I'd been offered to do some other shows but never felt the timing was right," she told me.
Since appearing on "American Idol" a dozen years ago, McPhee has worked steadily as a singer and actress, even managing to combine both talents on the TV Series "Smash," which was about the staging of a fictional musical.
"In some ways I got to live out both dreams of being on TV and Broadway, but after I got off that show I really wanted to be on Broadway for real," she said.
All of which led her to accept a role in "Waitress" and to meet Sara Bareille, who wrote the music for this show and has played the lead role just as McPhee does now.
"Pay attention to those pies" (that her character makes in the show) Bareilles told the new star as she got ready to go on.
"You can't help but have anxiety of like 'oh my gosh, oh my gosh, what if i can't do it?'" McPhee said.
Not to worry, the reward here more than justifies any risk, as McPhee has already realized.
"Feeling the energy of creating something, and getting the audience to feel something with you, is a really moving and powerful experience," she said.
Katharine says she has been too busy rehearsing for her Broadway debut to watch the new season of "American Idol" though she remains very proud of her sister - who works with the contestants on the show as a vocal coach.
It's the same job their mother did on 'Idol' for many years.