"Her story is not unlike so many other stories that I've heard or seen," said McCain, who has been involved in Congo since 1994. "These people have such incredible spirit."
With her years of experience in the country, McCain said she was at first skeptical about Affleck, fearing that he was like other celebrities that she'd seen who drop in to Africa for what she called a "parachute photo op."
"My past experiences with other folks in the same, perhaps, arena have not been good. It hadn't been heartfelt. It hadn't been, what I believe, truly committed to the issue and the people," McCain said.
But Affleck, she said, has done the hard work of learning the issues on the ground in detail.
"This has been not only a wonderful opportunity but a wonderful treat," she said. "Ben has a working knowledge of this region. He not only understands the issues, but he understands what we need to be doing."
And Affleck added that while the Democratic actor and the Republican political wife may be separated by a partisan divide, the two have found plenty of common ground on Congo, family and even politics.
"I'd say we have, actually, frankly, quite a bit in common. I don't know if, maybe she doesn't want me to say that out loud," Affleck joked.
Watch "World News with Diane Sawyer" and "Nightline" for more tonight. And watch them live Tuesday on "Good Morning America."