Dame Emma Thompson wrote Prince William a "fan letter" after he presented her with her damehood.
The 62-year-old actress was given her title in the 2018 Birthday Honours and she heaped praised on how "absolutely fantastic" the 38-year-old royal was at the investiture ceremony, even though he turned down her approach for a kiss.
Appearing via video link on "The Jonathan Ross Show", she said: "Prince William was absolutely fantastic. I wrote him a fan letter afterwards.
"His face and body language, so kind and generous to every single person. I've known him since he was little really. I've always liked him.
"I couldn't believe how tall he'd got. I did say, 'I suppose I better not kiss you' and he said, 'Probably better not.'
"I asked him about him and he said, 'Will you please stop talking about me, it's about you.' What a nice man."
Thompson also revealed she bonded with her "Cruella" co-star Emma Stone by drinking shots.
She said: "Who doesn't bond over tequila for crying out loud?"
Her co-star added: "I've been an enormous Emma T fan for as long as I can remember. I knew I was going to be in love with her and then she came through with flying colours.
"And the opposite of this character who is a complete narcissist, a brutal woman. Emma T is a polar opposite, so a real joy on and off screen."
Thompson previously admitted she and Prince William "just sniggered" at each other during the honours ceremony.
She said after the event: "I love Prince William, I've known him since he was little, and we just sniggered at each other.
"I said, 'I can't kiss you, can I?' And he said, 'No don't'!
"If you're first up you have to be more formal on such an occasion, but he's looking wonderful and doing so well.
"It's really lovely because I've always loved the boys and I've always been a long-term correspondent with their dad, it's a very lovely feeling."