The pair devised the game as a punishment for losing at dominos.
Whoever wins the dominos game has four paintball shots at the other – while they run around topless in the garden.
“It started where you could put on a jacket and everything, but we were like ‘the welts aren’t that bad when it’s like that’,” Wilson laughed.
“Then we reduced it to you could just wear shorts – that’s it.”
Wilson, 42, showed a video of his and Harrelson’s paintball challenge during an appearance on Jimmy Kimmel’s US chat show.
The hilarious video sees Wilson shooting paintballs at his pal in the garden of Harrelson’s Hawaiian home.
“I got a paintball gun for Christmas, but I only got one,” Wilson told Kimmel. “There’s only so long you can shoot at the trees and stuff.
“What we came up with, Woody and I, was playing dominos. The loser had to run for the other guy.'
He added: “The reason we had to start filming was because there was so much arguing over how many shots the other guy took. You’re only allowed four.”