Alley played Richards' mom in 1999's Drop Dead Gorgeous. On Tuesday, Richards, 51, called her late costar "one of the funniest, nicest & strongest women I've ever met" in an Instagram post showing a still of the two in the film.
"I am so lucky & grateful that I got to do a movie with her playing my mom," Richards wrote. "I was quite intimidated to work with her but she immediately made me feel so comfortable."
Richards said Alley "had a heart of gold & deep love for animals." She added, "Her laugh was infectious & she lit up every room she walked into. Kirstie you are loved by so many & will be so very missed. Prayers to your kids & grandkids."
In Drop Dead Gorgeous, Alley portrayed small-town beauty pageant organizer and former beauty queen Gladys, whose daughter Rebecca (Richards) competes against pageant contenders like Amber (Kirsten Dunst) and her own mother Annette (Ellen Barkin) in an increasingly dangerous series of competitions.
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Alley's children, True and Lillie Parker, announced the actress had died from cancer: "We are sad to inform you that our incredible, fierce and loving mother has passed away after a battle with cancer, only recently discovered."
"She was surrounded by her closest family and fought with great strength, leaving us with a certainty of her never-ending joy of living and whatever adventures lie ahead," they continued. "As iconic as she was on screen, she was an even more amazing mother and grandmother."
Gille Toucas/CBS via Getty, Chelsea Lauren/Variety via Getty Denise Richards and Kirstie Alley
They added how their mother's "zest and passion for life, her children, grandchildren and her many animals, not to mention her eternal joy of creating, were unparalleled and leave us inspired to live life to the fullest just as she did."
Other stars like John Travolta remembered Alley in social media tributes Monday. "Kirstie was one of the most special relationships I've ever had," he said of the late actress, sharing a throwback photo of Alley and a second photo of them smiling at one another. "I love you Kirstie. I know we will see each other again."