"Like riding a bike," Keaton replied before explaining how it's easier to move around in the suit if you're skinny as opposed to be being muscular.
"I don't know why I didn't think of this the first time. It's easier to be really skinny and not work out, because you have more room to work around," he said.
The 69-year-old then recalled how Jack Nicholson who portrayed The Joker in Batman, questioned why he was working out on the set on the film.
"I'm getting ready, I'm working on the bag and I'm doing all this stuff. Jack Nicholson walks by who was playing The Joker and he looks at me and he goes, 'What are you doing?' And I go, 'Eh, you know working out.' He goes, 'What for?' I stopped, sweating and I went 'I don't know," Keaton recalled.
"He just walked away. I thought, 'He's right.' Totally right because you got a suit. What do I need to do? The suit makes me look good," he continued.
The Flash, from It director Andy Muschietti, will hit theaters on Nov. 4, 2022. Ezra Miller returns as The Flash who will be traveling to different dimensions where he will meet other versions of DC's stable of characters.
Keaton and Ben Affleck will both be reprising their roles as Batman in the film with Sasha Calle as Supergirl.
Keaton stars in the new action film The Protégé, which comes to theaters on Friday.