Will Smith described showing Venus and Serena Williams "King Richard" for the first time.
"It was the worst two hours of my life," he said.
Smith plays Richard Williams, the father of Venus and Serena Williams, in the biopic.
Will Smith was one of the featured guests on "The Graham Norton Show" Friday, during which he described the uncomfortable experience of showing Venus and Serena Williams "King Richard," according to preview quotes from the show, which was recorded earlier, sent to Insider.
Smith - who produced the biopic and starred as the Williams sisters' father Richard Williams - said both Venus and Serena Williams had agreed to act as executive producers on the film and aid the production, "but they wanted to see the movie before they decided if they would put their names to it."
Discussing the first screening for the sisters, Smith said: "It was the worst two hours of my life, but they were a mess with tears, and they loved it."
Later in reference to his previous work on "Ali," the movie biopic of the legendary boxer Muhammad Ali, Smith said: "I had made the mistake of watching 'Ali' for the first time while sitting behind Muhammad Ali and mid-way through the movie, he turned to his wife and said, 'Was I that crazy?'"
"King Richard" debuted in theaters and on HBO Max November 19. The film, which was directed by Reinaldo Marcus Green and has already become an awards season frontrunner, follows Smith's Richard Williams as he fights to launch the professional tennis careers of his two young daughters from their home in Compton and charts how their ultimate success has helped to change the game of tennis.
Discussing his decision to take on the role of Richard Williams on "The Graham Norton Show," Smith said: "It was such an opportunity to dive into the life of this family and spend time uncovering their amazing story. He predicted Venus and Serena being the world's number one and number two women's tennis players two years before they were born! He wrote a 78-page plan. It was a really crazy prophecy and so amazing it's hard to comprehend."
Earlier this month, a report in The Hollywood Reporter revealed that Will Smith, who earned as much as $40 million for his work on the highly-anticipated biopic, gave personal cash-bonuses to his "King Richard" co-stars to compensate for the financial impact of Warner Bros.'s decision to release the film simultaneously on HBO Max and in theaters.
Speaking about the bonuses with Entertainment Weekly, Smith said it was his "honor" to compensate his castmates.
"All I can do is my part, so it's always an honor and a pleasure for me to do my part, and everybody put in brilliant work. It was the COVID shutdowns, people really stuck with us," he said.
"It was a much more difficult process than anybody thought they were signing up for. I just felt that it was fair."
"The Graham Norton Show" starring Smith airs on BBC One Friday, November 26, and will be available on BBC iPlayer.