"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 shared on Jimmy Kimmel's post-Oscar special on Sunday night.
The 43-year-old actor recalled having to "sign my life away" to borrow his Batman v Superman suit -- cowl and all -- for the afternoon.
The other superheroes at his son's party -- Wonder Woman, Iron Man and Captain America -- were well-equipped with prepared bits, but Affleck admitted the experience "was so mortifying" for him.
"First, I thought it's kids, it's kids! I didn't think that, of course, all their parents were there," he said. "So I have to put on my outfit from this movie and put on my scary face for the 4-year-olds and all their folks are there."
"It's 11 in the morning and they're kind of like, 'Wow ... Batman,'" Affleck continued, laughing at his own expense before switching to his deep, gravelly Batman v Superman voice: "This means a lot to my son! Don't make me come find you."
In attendance at the b-day party were the children of Garner and Affleck's famous friends, including Reese Witherspoon's son, Tennessee, who dressed up as Captain America.
Affleck's appearance on Kimmel comes days after Garner opened up for the first time publicly about their split.
"Sure, I lost a dream of dancing with my husband at my daughter's wedding. But you should see their faces when he walks through the door. And if you see your kids love someone so purely and wholly, then you're going to be friends with that person."
Affleck has yet to publicly comment on the article.
To hear more about Garner’s interview, watch the video below.