The former E.R. star took to the West End stage with Slater in 2006 and the pair became firm friends during their time spent rehearsing together.
But when the shows started, Slater was up to mischief – and Kingston often found herself stifling laughter during the performances.
She tells Britain’s Celebs on Sunday magazine, “We did One Flew Over The Cuckoo’s Nest together and had such a blast. He was so much fun and a fantastic actor.
“He has the most amazing stage presence, but was terrible for making me corpse (laugh on stage). There were times when I laughed so much I couldn’t continue.
“Luckily, the audience would see why I was laughing and laugh themselves. And then I had to pull myself together again. Sometimes I’d try to make Christian corpse, but all I ended up doing was making things worse for myself.”