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.
"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.
"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.