Culp seems a bit baffled that I Spy, which broke ground by pairing a white man and a black man as equals and pals, hasn’t been more firmly canonized in television history or—as he puts it—the history of all entertainment. “It had never been done before, not just in television, not just in movies, not in the theater either. There had never been a show ever of any consequence whatsoever with a black lead and a white lead who liked each other. There was Othello, but Othello didn’t even know that his nemesis hated him. But outside of Othello, there never was a black and white combination specifically, anywhere in the theater,” he contends. “It didn’t just break the ground for television. It broke the ground for everything, everywhere, forever.”

Wow, okay. True, what sets I Spy apart is the uncanny chemistry between Culp and Cosby, evident in the pair’s witty banter. The characters trade quips in a way that suggests a long-standing friendship, which clearly came from their friendship off the set.

While shooting the pilot, Culp realized the script was weak, so he took it upon himself to write episodes during the hiatus before full production (he already had some respected television-writing creds, including an episode of Cain’s Hundred that got a rare write-up in the L.A. Times). And, crucially, he and Cosby cultivated an offscreen friendship in daily phone calls. “I talked to Bill every single day without fail. He was all over the place doing [stand-up at] nightclubs in Canada and the East Coast. He would just stop wherever he was in the middle of the night, and we talked for at least an hour,” Culp says.

The pair were notorious for punching up their dialogue on set. “A lot of writers did not want to write for Bill and me because we messed with their words,” he says, “but we only did it because they weren’t good enough.” Culp takes justified pride in the scripts he wrote: He got an Emmy nomination for writing in 1967. That came on top of his noms for best actor in a dramatic series for each of the show’s three seasons. He lost the actor prize every time, but the sting was lessened by the fact that the winner—every time—was Cosby. “The only material that we never changed a syllable of was mine. Bill knew perfectly, while reading it, that it was gold, and we did them word-for-word. They were that well-crafted.”

When pressed as to why I Spy was able to showcase an interracial friendship and develop a realistic black character when both ideas seemed bold, Culp recalls talking to Cosby about it: “Bill said to me one day, ‘Our statement is a nonstatement.’ I said, ‘Absolutely. Our statement is a nonstatement.’ That is the secret to the success of this show—that and the fact that we had no watermelon jokes.”

I Spy: Seasons 1–3 are available now.