Star Trek: The Original Series was an early attempt at a diverse cast that would speak to Earth and humanity reaching a peaceful resolution of its many problems before heading to the stars. With the show’s second season, creator Gene Roddenberry wanted a younger crew member for audiences to identify with, someone that acknowledged the public’s appetite for groups like the Beatles and shows like The Monkees. This was a smart choice for a character addition. A show needs viewers, even if the eventual role was filled by an actor only a few years younger than William Shatner or Leonard Nimoy.
But thematically to the show, what else could be done to spruce up the new character beyond “Hey, here’s a young guy with a mop top” ?
Reflecting on the current crew would give an indication as to what made this show so special. The casting of Nichelle Nichols as Lieutenant Uhura spoke to progress on the racial front. The inclusion of Sulu, played by George Takei, addressed peace in Asia. Spock was an alien. But what about the Cold War?
Making the cast member a young Russian told the viewers that, yes, peace between the superpowers of the 20th century would be achieved. Roddenberry then wrote to Pravda, the official newspaper of the Soviet Union, to let them know that there would be Russian character in Star Trek. Somehow, perhaps a ploy for publicity, a rumor spread from the studio that the choice was somehow prompted by a Pravda article criticizing the show for not having a Russian crewmember.
Here’s a Snopes article on the topic: http://www.snopes.com/radiotv/tv/chekov.asp?_escaped_fragment_
The end result has to be laid at Roddenberry’s feet, as Chekov became the newest crew member of the Enterprise, played with exuberance by Walter Koenig for the rest of the show’s run and its many later films. The character’s popularity was felt immediately as demonstrated by the volume of Chekov fan mail.
The role would be reprised by Anton Yelchin for the three 2009-2016 reboot movies. His casting doubles down on the youthfulness of Chekov. Yelchin’s tragic accidental death in June, 2016 leaves the character in limbo. Currently there is no plan to put in a third actor in the role.