The former "Cheers" star has been open about her feelings for her "Look Who's Talking" castmate, admitting she would have jumped in to bed with Travolta if she wasn't married to actor Parker Stevenson at the time.
But she had to make sure her relationship with the "Grease" star was purely platonic when his new wife, Kelly Preston, confronted her about her behaviour around him.
"Kelly came up to me... and she said, 'Erm, why are you flirting with my husband?' " she said during an appearance on British podcast "The Dan Wootton" Interview. "That was sort of when I had to make a decision and that was pretty much the end of that."
Looking back, the actress is glad she and John never hooked up, adding, "John and I would have devoured each other because John and I are so alike.
"It would be like two blazing stars that just fizzled out. John goes to bed at, like, four or five in the morning and wakes up at three in the afternoon. I go to bed at nine o'clock at night and wake up at five in the morning. We basically would have not have seen each other. It would have been a disaster."
But both she and Travolta loved being close to each other: "John would agree it was mutual that we sort of fell in love with each other," Alley said, revealing not sleeping with her "Look Who's Talking" co-star was "one of the hardest things" she's ever done.
"I was madly in love with him," she told Wootton. "We were fun and funny together... It wasn't a sexual relationship because I’m not going to cheat on my husband. But, you know, I think there are things that are way worse than sexual relationships, than cheating on someone that way. I consider what I did even worse because I actually let myself fall in love with him and stay in love with him for a long time."
Alley and Stevenson were married from 1983 to 1997, while Travolta wed Preston in 1991.