Ability over Appearance: Originally an older actor was slated to play a fatherly mentor to Robert Culp's character. After seeing Bill Cosby performing stand-up comedy on a talk-show, Sheldon Leonard decided to take a chance on hiring him to play opposite Culp. The concept was changed from a mentor-protégé relationship to same-age partners who were equals.
Acting for Two: Robert Culp as Kelly and the General in "The Warlord".
Actor-Shared Background: Many details of Bill Cosby's life were written into his character. Scott does not drink or smokewhile Kelly Robinson does both. There are frequent references to Scott's childhood in Philadelphia and attending Temple University (Cosby is sometimes seen wearing his own Temple sweatshirt), and in the "Cops and Robbers" episode, Scotty returns home to Philadelphia to revisit his old neighborhood.
California Doubling: Unlike just about every other spy series of the time, averted. Although there was still some filming done in Hollywood, if you were watching an episode set in Rome, for example, you were seeing the real city of Rome and not the Desilu Studios backlot. This was mostly due to the producers wisely becoming friends with the crew from NBC News, who allowed them to travel along.
Throw It In!: Much of the banter between Cosby and Culp was ad-libbed, with the actors often tossing in references that left them Leaning on the Fourth Wall, such as Culp, in one episode, joking about how he never wins awards in a scene filmed soon after Cosby won an Emmy in a category for which Culp was also nominated. Cosby also occasionally made references to events and characters in his comedy routines.
What Could Have Been: Producer Sheldon Leonard made concerted efforts to arrange for filming on location in both China and the Soviet Union. According to books on the making of the series, China agreed to let him film at the Great Wall and Forbidden City, but the deal fell through when the country required its film processing facilities be used, which were judged to be of too low a quality for US TV broadcast; Russia came very close to a deal until the US government objected. According to Leonard the plan for Russia was a storyline in which Scott and Robinson teamed up with Russian agents to fight a common threat (this a decade before the Bond film The Spy Who Loved Me did the same concept). After Russia fell through, a set of episodes filmed in the San Francisco region were made instead.
Milestone Celebration: I Spy Mystery, which celebrates the series' 10th anniversary, and I Spy Spectacular, which celebrates the series' 20th anniversary.
I Spy Mystery was actually one of the original books from the 90's but it did get an entirely different cover for the 10th Anniversary, making the original edition of the book with the black cover a lot more rare and relatively more pricey than other older books.
Recycled Script: Well, not so much "script", but the Challenger books all reuse pictures from the previous books with newer (and usually much more difficult) riddles.
The video games:
The Other Darrin: An entirely different set of children voice the riddles for Craft Projects in School Days from all the other themes. This is mostly because the original I Spy game excluded this theme, and presumably the several years long gap required the developers to find new children to voice the riddles.