Walter Koenig was upset about how he found out he wasn't in Star Trek: The Animated Series

When Star Trek: The Animated Series debuted in 1973, it was missing one character who'd been on the bridge for two of the three seasons. Walter Koenig's Ensign Pavel Chekov wasn't part of the crew, which, most likely came about because of budgetary reasons. Originally, Nichelle Nichols and George Takei weren't going to be on the series, either. Their characters were, but they were going to be voiced by other people until Leonard Nimoy said he wouldn't do the series unless Nichols and Takei were hired since they had been there from the start.

Koenig, however, came onto the series in the second season, and wasn't invited back for The Animated Series. According to The Fifty-Year Mission—The First 25 Years by Edward Gross and Mark A. Altman, Koenig was upset with how he found out he wasn't going to be a part of the show. Instead of being called and notified, he learned the news at a convention.

"I was upset with the way I found out that I wasn't a part of the show—at a convention. Everybody thought someone else had told me apparently. Dorothy [Fontana] thought Gene had and Gene thought Dorothy had."

- Walter Koenig

This was definitely an instance of the ball being dropped. Koenig was in thirty-six episodes of The Original Series out of the seventy-nine episodes filmed, and having been there through the end, it's certainly understandable why he would expect to be included in The Animated Series. Instead, though, Chekov was nowhere to be seen in the series, yet, he returned to the character in Star Trek: The Motion Picture.

Money aside, after being rejected in such a rude manner, it's a wonder Koenig wanted anything more to do with Star Trek. But, in all reality, we can't put money aside, and that's the draw for any actor. Fortunately, Koenig wasn't left out of future movies.

Star Trek: The Animated Series ran for two seasons, totalling twenty-two episodes before being cancelled in 1974. It is still beloved by Star Trek fans and was the first Star Trek series to win an Emmy in 1975 for Outstanding Entertainment – Children's Series.