"I think of how sweet Keanu Reeves was and how handsome he was," Bullock recalled of filming.
"It was hard for me to like, really be serious," she said. "Like, he would look at me and I'd be like [giggles]. And I'd have to try, you know [be serious]."
Bullock said Reeves was particularly considerate as they filmed an action scene.
"My dress sort of kept flying up," she told host Ellen DeGeneres. "I said, 'Whatever you do, just keep my dress down.' The whole stunt, he just made sure my dress [was down]."
Bullock told DeGeneres she never dated Reeves but remains friends with the actor to this day.
"Never dated him. There was something about me that I guess he didn't like," she said with a laugh. "I think we're probably friends for that long because we didn't."
Bullock and Reeves went on to star in the 2006 movie The Lake House. Bullock told DeGeneres in May that Reeves was her favorite on-screen kiss.
Bullock stars in the new Netflix movie Bird Box, which debuts Friday. Reeves, meanwhile, will reprise John Wick in John Wick: Chapter 3 and have a voice role in Toy Story 4.