Affleck sat down with Jimmy Kimmel on Tuesday's Jimmy Kimmel Live! and he opened up about his son Samuel's recent 8th birthday, which they all celebrated last week.
While Affleck was in London shooting his upcoming historical drama, The Last Duel -- alongside co-stars Matt Damon and Driver -- he made sure to fly back to Los Angeles to be with his son on his special day.
"I'm a divorced dad, I get half the time with my son, and it was his birthday! So I said to [the producers], 'I'll do whatever the schedule needs, but on the 27th, I gotta be in L.A.'" Affleck explained. "So they set it up where I was shooting right up to the 27th, and then I kinda had to scramble… and I was going to land right in time for his birthday."
"We had presents set up and everything was getting shipped and sent, and I was gonna pick it up and go to his surprise party," the actor added.
Backing things up, Affleck shared a small but crucial detail about his 8-year-old son: He understands that what his dad and mom do -- as actors -- isn't real. They play make-believe roles and get paid for it.
"But he also knows that Star Wars is real," Affleck said with a laugh, explaining that his son 100 percent believes that the events of the intergalactic action drama are completely factual -- which means Affleck's experience working alongside Driver had a special significance for his son.
"I told him, 'I'm actually in this movie with Kylo Ren,'" Affleck shared, adding that Samuel's mind almost imploded at how big a deal that was, and how much his son absolutely loved Kylo Ren.
Getting back up to speed, Affleck recalled how, when he got off the plane after an 11-hour flight, and he got home, he knew something was wrong when he saw the look on the face of one of his assistants.
"I said, 'Where are the presents?' And he said, 'Well, they didn't get here,'" Affleck recalled. "So it meant I had to show up to my son's party with no gift for him. And there was this sort of sinking, awful feeling."
"And then he goes, 'But, there is something! Adam heard it was your son's birthday. So he called your assistant and got your address and sent some presents, and signed a card and a picture from Kylo Ren,'" Affleck continued. "So I took those presents and went to see my son."
"I watched him open all the other presents, and then I said, 'Sam, my presents didn't get here on time. But I did get a present from somebody who really wanted to make sure you got a gift,' and he was like, 'Who?' And I said, 'Kylo Ren,'" Affleck shared.
"He opened the presents and I played him the video, and it was an incredibly moving and powerful moment," the adoring dad said, as his voice cracked with emotion. "Adam made me a hero to my kid, and I will never, ever, ever forget that."
The two-time Oscar winner explained that the story is "a really good lesson in doing those small gestures of kindness."
"Because you have no idea," he said. "For him [it was no big deal], but it meant the f**king world to me. It meant so much."
Before Affleck, Driver and Damon take on the roles of French nobility for the Ridley Scott-directed historical dramaThe Last Duel, fans will get a chance to see Affleck playing a recovering alcoholic who gets a chance to turn his life around when he's given the opportunity to coach the high school basketball team at his alma mater in The Way Back.
ET sat down with Affleck last month, during his press tour for the emotional sports drama, where he explained why he saw his personal experience with the topic as "an advantage" when making the flick. He added that he was "feeling a full range of kind of access to my emotions" and felt "ready to do a heavy, performance-based piece."
"I knew that in this day and age of celebrities' personal lives becoming news stories, those were questions I was kind of going to answer anyway," he said. "The interesting thing for me was to be able to define the story myself, the way I see it, which is really one of hope."
"I know and have a lot of friends who have dealt with issues like this compulsive behavior and addictive behavior, and the vast majority of them are really honest, accountable people living good, healthy lives," Affleck added. "The idea that life gets better, that you can get better, that you can overcome your obstacles, is a really important one to me and that's the approach I liked about this movie. It was not just like, 'Oh, there is alcoholism.' That's kind of ordinary."
Affleck even described the making of the movie as containing "a certain amount of therapy" due to its "emotional honesty."
Hear more from Affleck in the video below.