The second season of Star Trek introduced fans to Ensign Pavel Chekov, who would permanently fill what had previously been the rotating position of ship’s navigator. With an eye toward attracting more teenage girls to Star Trek’s audience, Gene Roddenberry had decided to add another character to the bridge crew: someone young and slightly irreverent, someone Beatle-ish… or at least Monkee-ish. But a timely snipe against the series from the official newspaper of the Communist Party would transform Roddenberry’s vision of a cheeky Brit into a humorously ethnocentric Russian.
Star Trek, it seems, had leaked through the Iron Curtain. An article appeared in Pravda slamming the “typically capitalistic” mindset behind the show. Why, the paper wondered, were there no Russian crewmen aboard the Enterprise? Were Americans so ill-informed about the achievements of the Soviet space program?
It was a valid point and an unintentionally clever suggestion. In the same way that the presence of Uhura and Sulu served to demonstrate that integration would be commonplace in the not-so-distant future, the presence of a Russian officer would demonstrate that petty disagreements between Earth’s nations would become a thing of the past. […]
Thus, Walter Koenig was cast in the role of Ensign Pavel Chekov. Roddenberry still liked the idea of the character sporting a Beatles-style haircut, even if the character had a Muscovite accent, so the actor wore a mop-top until his own hair grew to an appealingly shaggy length.
source: Star Trek: The Original Series [Paula M. Block with Terry J. Eroman, 2010]