Many funny actors will go an entire career without landing a lead role in a television sitcom. Meanwhile, Kaitlin Olson is holding down two of them simultaneously: Since 2005, she has played the beleaguered bartender Sweet Dee on “It’s Always Sunny in Philadelphia,” the proudly unrefined ensemble comedy that begins its 12th season on Wednesday, Jan. 4, on FXX.

And on Sunday, Jan. 1, she takes center stage in the new Fox comedy “The Mick,” playing its title character, Mackenzie Murphy — a.k.a. Mickey — a bad seed who must abruptly become a surrogate parent to her niece and nephews when her wealthy sister and brother-in-law flee the country to avoid federal fraud charges. Now she is unwillingly raising three children she never wanted, in a suburban mansion she could surely get used to.

“The Mick,” which Ms. Olson works on in the months when she is not making “It’s Always Sunny,” is hardly a side project; she is also an executive producer on the Fox show, where she helped to hire the cast and writing staff, and gives notes on scripts and editing. “That triples your workload,” said Ms. Olson, who has two sons with her husband, Rob McElhenney, the creator and co-star of “It’s Always Sunny.”

“My children have trained me well for multitasking,” she added.

In a rare moment of down time from these responsibilities, Ms. Olson spoke by phone about what she’s learning she can and cannot get away with on Fox. These are edited excerpts from the conversation.