Kirstie Alley Mourned by 'Cheers' Stars Ted Danson and Kelsey Grammer

Kirstie Alley's death is not only shocking to fans, but also to her closest friends and costars -- like Ted Danson and New Kids On the Block singer Jonathan Knight -- who are expressing their love and admiration for the late actress.

Her fellow "Cheers" vet, Ted, says he was actually on a plane Monday, watching an episode of their classic sitcom -- the one where guest star Tom Berenger proposes to Kirstie's character, Rebecca.

He told People he got the horrible news about Kirstie when he got off that flight.

He added, "I am so sad and so grateful for all the times she made me laugh. I send my love to her children. As they well know, their mother had a heart of gold. I will miss her."

Kelsey Grammer also shared some of his thoughts on the loss, saying ... "I always believed grief for a public figure is a private matter, but I will say I loved her."

Of course, "Cheers" fans know Kirstie got her big break starring alongside Kelsey and Ted on the show.

Jonathan, a longtime friend of Kirstie who she'd jokingly refer to as "her husband," expressed his pain too ... posting on social media about how his "heart is ripped into a million pieces, and half my soul has been taken away."

Kirstie's final TV performance was actually in April on "The Masked Singer" -- she competed as the Baby Mammoth, beltin' out to "Walkin' After Midnight" by Patsy Cline and "The Shoop Shoop Song" by Cher.

She got a standing ovation from the judges, and said she wanted to be on the show for her grandkids.

As we reported, Kirstie's family announced her death Monday, saying "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 71.