'Mom' star Allison Janney coming to terms with series ending: 'Biggest emotional rollercoaster'

Janney also discussed Mom's eight-season run on CBS coming to a close. 

"Sometimes, I'd be driving on the lot, and it was like, 'Oh my God, this is the last Monday I'll be driving on the Warner Bros. lot,'" she said, adding: "I'd come on and just everything would make me cry, and then the next day, I'd come on and feel like, 'I'm so excited this is ending! I get a new chapter of life!' It was the biggest emotional rollercoaster ride."

Mom, partially created by Chuck Lorre , debuted in 2013. The comedy sitcom centers around Bonnie Plunkett (Janney) and her estranged daughter, Christy Plunkett (Anna Faris), as recovering addicts. The earlier seasons focused on their home life, with Bonnie moving in to help Christie raise her two children, but the storyline eventually fully leaned into Bonnie and Christie's Alcoholics Anonymous support system—Marjorie (Mimi Kennedy), Wendy (Beth Hall), Jill (Jaime Pressly). Octavia Spencer and Kristen Johnson have had recurring roles.

Faris departed the series following the seventh season, announcing the move last fall. Her Christy character was written off in the Season 8 premiere with Bonnie and her husband, Adam (William Fichtner) dropping her off at the airport to go to Georgetown Law School. Faris did not appear in the episode.

With Fallon, Janney foreshadowed the next chapter in her storied acting career.

"As I like to do all the time, whatever I've done, the last thing I've done, I like to shake it up and do something completely different," the 61-year-old said. "So, I've decided, Jimmy, that I'm going to go do an action thriller movie, and I'm about to start training with a fight trainer two hours a day And learn to fight to kill and be a bada**, like, female Dirty Harry in this movie called Lou that I'm about to start filming."

Per IMDb, Lou will have Anna Foerster as its director and Maggie Cohn as its writer.