The 43-year-old actor, who will play Batman on screen in upcoming film "Batman v Superman: Dawn of Justice", said he found it "mortifying" dressing up as the edgy superhero in real life, reported Female First.
"He wanted a superhero birthday party and he wanted superheroes there, and I was like, 'Alright, what heroes do you want?' He's ticking off heroes and he was like, 'I want you to do Batman ... Real Batman!'" Affleck said.
"There was Wonder Woman, Captain America, Iron Man and there was me... It was so mortifying."
Affleck was so embarrassed because he had forgotten all the young guests would be accompanied by their parents.
Speaking on Jimmy Kimmel's Oscars special, he said, "First I thought, 'It's kids. It's kids.' I didn't think that, of course, like, all their parents are there.
"So I had to, like, put on my outfit for this movie, and put on my scary face for the four year olds and all their folks are there. It's 11 in the morning and they're kind of like 'Wow. Batman.