Rock said Reigns was the first name they thought of when they discussed who could play his brother in the movie.
"As we were putting together the story of Hobbs & Shaw, we thought who can be my brother? Obviously the first name that popped up was Roman Reigns," Rock said.
"It was a crazy process for me. I read for the role," Reigns said.
"Let me just be clear to everybody - he read for the role because we don't believe in nepotism. We do believe in taking care of each other, but he's got to earn it," Rock added.
The Rock also praises Reigns for being versatile and adapting to the movie set.
"There are a lot of qualities about Joe [Roman] that make him appealing to a global audience," Rock said. "Obviously it shows when he's able to make a transition like this, and the very first day, first take - bang! He knocked it out of the park. It was a very great feeling because then I was like, 'Mic drop, b-----s. Look at my cousin.'"
Reigns added, "To be out here in Kauai, to experience this Mana, has been overwhelming in the best way, and since you're right here, thank you."
"Ah well, of course," Rock responded.