Something occurred to me just this morning about TMP and the command staff. When trapped in the wormhole, Kirk orders phasers and Chekov complies, but Decker countermands that order in favour of torpedoes, as the phasers were inoperative with the warp drive being off-line. Why didn’t Chekov suggest torpedoes? Admittedly Decker was on the ball and got in there quickly, but surely Chekov would’ve at least started to mention phasers being useless—or at least know to use torpedoes instead, despite Kirk’s order. Does that mean the Chekov wasn’t originally intended to serve as Security Chief so, like Kirk, wasn’t aware of the redesigned weapons array? Was he (and possibly Sulu and Uhura) not part of the original relaunch crew, but rather added at the last minute with the launch of the ship ahead of schedule?
We might well assume none of the original heroes were going to be part of the relaunch, until Kirk intervened. Scotty just built the new ship, and the others were doing other things altogether before they came aboard at the last minute, not yet knowing that it was all thanks to Kirk. For all we know, too, Chekov was indeed the Chief of Security - meaning he knew zip about the new big guns, just like Chief Giotto never was involved in firing the torpedoes in TOS. That would have been the task of the Chief of Gunnery or whatever, but the poor sap, gal or BEM didn't get aboard (or at least onto the bridge) in time. Timo Saloniemi
If Chekov didn't know the ships full weapons capabilities, why not let the Rhaandarite security/tactical ensign take the controls? His loyalty to Decker hints that he's been onboard for a little while at least, so would know the ships capabilities, leave Chekov either down below with his teams or at the internal security or damage control stations on the bridge.
Might be his silver shield also referred exclusively to shipboard security duties. At departure from Earth, the big guns presumably wouldn't be needed, so the person with the appropriate department colors (whatever those are) could have been working elsewhere. It's not as if the guns should have been any more finished than the warp drive, after all - there could have been plenty of work remaining belowdecks! Timo Saloniemi
I have heard a theory put around that Sulu, Chekov, Uhura etc may have only been there originally to 'hand over' the ship to their successors, a kind of ceremonial passing of the torch, as well as maybe facilitating the training of the Refit-crew, but that when the V'Ger crisis came up, Admiral Kirk bootstrapped them all into joining the mission, as they were all on board the ship anyway (not that it needed much persuasion on his part). One note I might add is that even if this is the case, I'd make an exception for Scotty -- I'd be reasonably confident to suggest that he, if anybody from the pre-Refit crew, would be the most likely to have stayed aboard the ship for her next tour of duty, especially if he'd had as much of a hand in her Refit as is implied.
In that case, they were there a bit early, weren't they? The ship wasn't supposed to launch until weeks later. ...Assuming he even was aboard her to begin with. He was working at the dockyards involved in the refit , but that might be the exact opposite of him being at liberty to join the missions of the ship. Starfleet might rather want him refitting the next ship, and doing it better thanks to all the lessons he learned the first time around. Timo Saloniemi
I like the idea of them there for a hand over and training regime, which would be enough reason for them to be there early. Given Scotty's familiarity with the ship I think it'd be safe to assume that he was overseeing the refit and would either remain onboard for at least her shakedown or be rotated onto the next Constitution-Class due for refit. Aside from him the only other members of the original crew that looked as though they were assigned to the new mission were Chief Rand and Doctor Chapel.
I'd always just assumed that Chekov was thrown by having Kirk back in command that he was just automatically following orders. Given the positive reaction by the bridge crew (save that Rhaandarite) when Kirk assumed command, I figured that Chekov agreed with Uhura's statement that their chances of survival just doubled. So, later, during a crisis, Kirk gives an order and Chekov's stress makes him just automatically follow it without thinking it through. After all, that's what he's used to doing in a crisis, since on-screen evidence indicates that, prior to the events of TMP, Chekov only served under Kirk. The kid grew up implicitly trusting Kirk's orders and wisdom! Kirk says, "Fire the phasers," and Chekov's thinking, "Yeah, right, the phasers! The keptin always knows what to do!" Meanwhile, Decker is the only member of the senior staff not awestruck by Kirk and he has the wherewithal to countermand the order. Chekov probably felt dumb afterwards, but I don't think he was ever ignorant of the Enterprise's capabilities.
I think the simplest answer is Decker chimed in so fast Chekov didn't have time to respond "But keptin..."
It was for dramatic effect. In any kind of reality, internal security and external tactical wouldn't even be a combined job - too big. Although, if one person DID hold both jobs at once, Worf would be that guy.
The movies don't force us to believe that Security and Tactical would be combined. All the missions we see are ad hoc improvisations with irregular crew complements or then otherwise fail to demonstrate who is firing the big guns. Chekov being Security in ST:TMP has the downside that he never was anything of the sort in TOS, and appeared in Science colors in the later movies. Plus he knows next to nothing about Security in ST6, another mission full of improvisation and irregularity and people performing in unlikely roles. Chekov being Tactical in ST:TMP also comes out of the blue, but at least it isn't in conflict with him knowing nothing about phaser alerts in ST6, and agrees with him crewing the guns in ST2. As for the colors of the badges in ST:TMP, those were never properly explained on screen, and our best bet might be to ignore original intent on basically all of them. The idea that Security and Tactical would be combined might have occurred to those making ST:TMP. Or then not. It's a feature of TNG after Tasha Yar's demise, and of VOY after the horrid casualties taken in "Caretaker", but is not seen elsewhere in Starfleet, not even aboard the small Defiant. Timo Saloniemi
This. Plus, aside from Kirk, Spock, and McCoy, I never got the impression that the TOS cast was ever really a true expert of their positions. The writers just seem to use the minors as familiar faces to bounce dialog off of. It doesn't seem to me that they started considering Positions = Knowledge & Experience -> Dictates Dialog until TNG; not abnormal for the evolution of TV/ movies in general.
That was Billy Van Zandt's reasoning, too, but as little more than a credited two-line extra at the time, he had little/no sway with Robert Wise. He was, after all, the "Internal Security" officer and should have been the one to notice the arrival of the Ilia Probe in Ilia's sonic shower. Indeed, Walter Koenig mentions, in "Chekov's Enterprise", the withering look he received when asking if Chekov was getting a closeup. The Rhaandarite only gets to collect Shatner's field jacket because Shatner suggested it. Van Zandt eventually jokingly asked Harold Livingston if the Rhaandarite could get zapped by the Probe, 'cos he had so little to do. Re Chekov, I think he'd only just arrived back from his Security training. Uhura and Sulu were probably still with the ship since TOS/TAS. (There is fan speculation that Sulu might have been intended to be Decker's first officer.)
...Hopefully with an associated promotion, as he'd be our first XO who explicitly doesn't hold the second-highest rank aboard the ship, or at least evenly compete for one with others. Or was Sonak, with half a braid more than Sulu, not in the actual chain of command, unlike Spock in TOS? Timo Saloniemi
I think this is the best answer, in universe or not. There wasn't time for Chekov to do anything before Decker countermanded the order. Chekov could have said, "I was doing that anyway" but why would he?
I like to think that Sulu was chief of operations - he's too senior to be 'just' a helmsman. Uhura was also clearly meant to be there. She was co-ordinating a lot of the re-fit work. Chekov was just a popular character who had no defined niche. He was often made to appear stupid for comedic effect even though he could also be portrayed as very observant and knowledgeable. He was an educated idiot. The problem was he could never be smarter than Spock so he often just looked like a simple idiot. Rand and Chapel also suffered from not having a clear niche of their own. Chapel was never allowed to climb out of McCoy's niche despite opportunities and Rand's considerable personality was never really brought to bear in the movies (e.g. being front and centre characters at Starfleet command in later movies).
But Sonak was perhaps also a last-minute appointment, "based, I'm told, on your recommendation, Admiral...", when it was clear that the Enterprise had to be raced out of spacedock early, and Kirk had recommended a Vulcan on board (because Spock was unavailable).