Kirstie had absolutely no ego onscreen, which is what made her so watchable on Cheers and so many other things, including an up and down movie career. Laurie Metcalf is often the same on Roseanne/The Conners; her visceral revulsion with herself after sleeping with Tom Arnold's character on that show is hysterical and reminded me a lot of Kirstie. It's funny that her other iconic role, Saavik in Star Trek, is so controlled and razor-precise, a total opposite of her typical wheelhouse, and yet she played both extremes (plus the steely noir antiheroine/villain in Carpenter's Village of the Damned that I mentioned in the edit above) perfectly.
Then again, Saavik was an interesting Vulcan character for Trek. The dialogue didn't make it into most cuts of TWOK, but Saavik was actually half-Romulan, which meant she had access to emotions. So it makes sense for a little bit of the other Kirstie Alley to slip into that very different character, when in her first scene, facing impossible odds, the controlled Vulcan officer mutters 'damn.'