The 55-year-old actor said one of the directors, Peter Farrelly, had been his roommate and wanted to cast him in the role, that later went to actor Jeff Daniels.
But Harrelson wanted to make things interesting. So, the "No Country For Old Men" actor challenged the filmmaker to play a game of pool, reported DigitalSpy.
"It was shot by one of my best buddies - my one-time roommate, in fact - Peter Farrelly, along with his brother Bobby. They'd had no success at that time, but really wanted me to do the movie.
The actor eventually won the game.
"Dumb and Dumber" also starred Jim Carrey.