Michael Keaton recently spoke about working out for his role as Bruce Wayne in DC's Batman, stating that he "approached it totally wrong" after hearing Jack Nicholson's reaction. Batman was one of the earliest blockbuster superhero movies, turning the genre into a magnet for actors looking to get a big break in Hollywood. Today's actors now tend to put a lot of effort into embodying their comic book roles. This was also the case for Michael Keaton, who engaged in a workout regime to better convey Bruce Wayne's build.

https://screenrant.com/michael-keaton-batman-1989-workout-training-jack-nicholson-reaction/