Chekov was the Navigator for the Second and Third Season (although Discovery and SNW seem to have done away with that position completely).He had a gunner/chief of operations role in Star Trek: The Motion Picture. Before that (and not until the second season, as PJO pointed out), we only saw him at Ops.
(First season was Mitchell, Bailey, Farrell, Riley, Stiles, Painter, DePaul, DeSalle, and a few others sitting in the Navigator's chair.)
He was a full Lieutenant by the time of TMP and was the Enterprise Security Chief and manned the Weapons Station (which I guess would be referred to as the Tactical Station now).
Believe or not, TOS did have a few different Security Chiefs though.
Sometimes it was a mute extra, other times they were speaking roles like Lieutenant Commander Giotto or Lieutenant Dickerson.
As for this week's episode,
OH MY GOD! THEY KILLER HEMMER!!!!
But at least it was a good death, sacrificing himself for his shipmates.
Not wild with what they're doing with Spock and the jury's still out on how the Gorn will shake out.
I have to say, they moved awfully fast compared to Arena.