"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.
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Ben Affleck says dressing up as Batman for his son's birthday party 'was so mortifying'
WASHINGTON, DC - MARCH 26: Actor Ben Affleck, founder of the Eastern Congo Initiative, listens to testimony during a Senate Appropriations Committee hearing on Capitol Hill March 26, 2015 in Washington, DC. The committee is hearing testimony on diplomacy, development, and national security in regards to Africa. (Photo by Mark Wilson/Getty Images)
US actor Ben Affleck testifies on issues in the Republic of Congo during a US Senate Committee on Foreign Relations hearing on Capitol Hill, on February 26, 2014. AFP PHOTO / Saul LOEB (Photo credit should read SAUL LOEB/AFP/Getty Images)
LOS ANGELES, CA - JANUARY 07: (EDITORS NOTE: This Image was taken in B/W color not availabe). Actor Ben Affleck, backstage during The 41st Annual People's Choice Awards at Nokia Theatre LA Live on January 7, 2015 in Los Angeles, California. (Photo by Frazer Harrison/Getty Images for The People's Choice Awards)
CENTURY CITY, CA - FEBRUARY 14: Honoree Ben Affleck accepts the Valentine Davies Award onstage at the 2015 Writers Guild Awards L.A. Ceremony at the Hyatt Regency Century Plaza on February 14, 2015 in Century City, California. (Photo by Alberto E. Rodriguez/Getty Images for WGAw)
SAN DIEGO, CA - JULY 26: Actors Ben Affleck (L) and Henry Cavill attend the Warner Bros. Pictures panel and presentation during Comic-Con International 2014 at San Diego Convention Center on July 26, 2014 in San Diego, California. (Photo by Kevin Winter/Getty Images)
HOLLYWOOD, CA - NOVEMBER 14: Actor Ben Affleck accepts the Hollywood Film Award for 'Gone Girl' onstage during the 18th Annual Hollywood Film Awards at The Palladium on November 14, 2014 in Hollywood, California. (Photo by Kevin Winter/Getty Images)
WASHINGTON, DC - FEBRUARY 26: Ben Affleck, former U.S. Sen. Russ Feingold and U.S. Secretary of State John Kerry speak during a photo op at The State Department on February 26, 2014 in Washington, DC. Affleck, founder of the Eastern Congo Initiative, will testify before the Senate Foreign Relations Committee about the war torn African country. (Photo by Kris Connor/Getty Images)
CENTURY CITY, CA - JANUARY 25: Director Alfonso Cuaron (L), winner of the Outstanding Directorial Achievement in Feature Film for 2013 award for ÂGravity,Â and actor-director Ben Affleck pose in the press room during the 66th Annual Directors Guild Of America Awards held at the Hyatt Regency Century Plaza on January 25, 2014 in Century City, California. (Photo by Frazer Harrison/Getty Images)
LAS VEGAS, NV - SEPTEMBER 18: (L-R) Singer/actor Justin Timberlake, actress Gemma Arterton and actor/director Ben Affleck introduce the world premiere of Twentieth Century Fox and New Regency's film 'Runner Runner' at Planet Hollywood Resort & Casino on September 18, 2013 in Las Vegas, Nevada. (Photo by Ethan Miller/Getty Images)
CULVER CITY, CA - JUNE 08: Actor/director Ben Affleck poses with members of the American armed forces at Spike TV's Guys Choice 2013 at Sony Pictures Studios on June 8, 2013 in Culver City, California. (Photo by Jason Merritt/Getty Images for Spike TV)
Ben Affleck arrives for the 2013 Vanity Fair Oscar Party on February 24, 2013 in Hollywood, California. Affleck's Iran hostage drama 'Argo' won the coveted best film Oscar late Sunday, while Daniel Day-Lewis took a record third best actor prize at the 85th Academy Awards, Hollywood's biggest night. AFP PHOTO / ADRIAN SANCHEZ-GONZALEZ (Photo credit should read ADRIAN SANCHEZ-GONZALEZ/AFP/Getty Images)
CULVER CITY, CA - JUNE 08: Actor/director Ben Affleck sits in the audience during Spike TV's Guys Choice 2013 at Sony Pictures Studios on June 8, 2013 in Culver City, California. (Photo by Jason Kempin/Getty Images for Spike TV)
HOLLYWOOD, CA - FEBRUARY 24: Actors Jennifer Garner(L) and Ben Affleck arrive at the Oscars at Hollywood & Highland Center on February 24, 2013 in Hollywood, California. (Photo by Jason Merritt/Getty Images)
LOS ANGELES, CA - DECEMBER 02: Ben Affleck is seen at 'Jimmy Kimmel Live' on December 02, 2015 in Los Angeles, California. (Photo by RB/Bauer-Griffin/GC Images)
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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.