The Oscar-winning star appears as Nick’s mum in the drama, which follows the lives of Charlie Spring and Nick Nelson after they meet at school and fall in love.
Since the series’ debut on the streaming service earlier this month, fans have celebrated the touching moment in which Nick comes out to his mother as bisexual.
However, Olivia was so moved by the scene as the pair prepared to film it that she cried, Kit told ITV’s This Morning.
Speaking to hosts Holly Willoughby and Phillip Schofield, he said: “We normally before every scene read it through about 50%, you don’t go 100% of the way. We were reading it through and suddenly Olivia just started crying.
“At first, I was thinking, ’God, she’s just really good, I need to step it up my game massively here,′ so I was really worried at that point. I think then other people told me afterwards that it was because it’s such a beautifully written scene and a crucial part in so many people’s lives. I think it really touched her.
“I was just scared to be honest,” he joked, describing her as “the most absolutely lovely person in the world”.
Heartstopper’s screenwriter Alice Oseman previously admitted she was “shocked” that the show had managed to get Olivia on board for her cameo appearance.
She told PA: “When we were discussing trying to get a big name to be Nick’s mum, I was literally like, there’s no way this is going to happen.
“This is the silliest idea ever and there’s literally not any point doing this.
“But we tried anyway and Olivia wanted to do it, and I just couldn’t believe it.
“There’s really no other way to say it, I was just shocked but so, so happy, because she’s such an incredible actress, she brings so much to that character, and all the scenes with Nick’s mum just feel so special and subtle and just really beautiful performance.”
This Morning airs weekdays at 10am on ITV.