"I wasn't really interested," Ritchie said of Hunnam in the starring role, "but his enthusiasm, let me tell you, something about ... his enthusiasm for the role," is what led the British actor to eventually be cast.
"I heard the guy was doing King Arthur and I promptly threw my name in the hat," Hunnam said, "And Guy threw it back out."
"I jumped on a plane and showed up on his doorstep and said, 'Let's have a cup of tea,'" Hunnam added. "By the end of that cup of tea, I think he decided he quite liked me, so he let me audition and I finally got the job. So it was a happy tale in the end."
Ritchie told "GMA," that Hunnam "was completely disinterested that I was disinterested in him."
The director said that trying to modernize the legendary story was "challenging."
"I had a two week existential crisis, where I'm convinced I've made the worst mistake in the world," Ritchie said. "And then the phoenix of confidence rises out of the ashes. And then gradually you find your momentum the film finds its own way."
Ritchie added that he hopes to be able to make a "King Arthur" sequel with Hunnam for fans.
"King Arthur: Legend of the Sword" will be in theaters nationwide on May 12, 2017.