The actor appears alongside Dakota Johnson in the franchise; the final part of which – Fifty Shades Freed – came out earlier this month.
Keaton has quipped that watching the BDSM-based moments in the movies gave the star an intimidating edge when she crossed paths with him in real life.
She said during an NBC TV appearance that “he’s a very nice looking man”.
“He’s kind of scary, too,” she added,” because of what he does to the girl – the Dakota girl.”
Johnson’s real life partner, Chris Martin, also seemed to catch her eye, when they crossed paths at Ellen DeGeneres’s 60th birthday party.
“I’m gonna talk about Chris Martin. He was with Dakota, right – they’re a couple. Got it?” she said.
“So, what I’m saying is – he’s gorgeous. That face got better with time.
“He was wearing a hat like this – oh, it was to die for.”
Fifty Shades Freed performed reasonably well at the box office; winning its opening weekend but quickly slipping upon the release of Black Panther.
It received a mauling from critics, even more so than its maligned predecessors in the trilogy.
Fifty Shades Freed is out now.