Question: In *Star Trek: Generations*, the actor Tim Russ plays an unnamed bridge officer (probably tactical/operations, as he's monitoring the ship's hull and systems integrity) on the Enterprise-B in the prelude. No pointy ears or eyebrows, so I must assume this character is not Vulcan and therefore not the Tuvok character that this same actor portrays in Star Trek: Voyager.

Was there ever an in-universe explanation given for this actor's appearance as someone other than Tuvok? Obviously any such explanation would be a retcon, as Generations was released a year before Voyager and so Tuvok's character didn't exist. But was there even an attempt, or is the role just dismissable as a human who happens to look similar to a notable Vulcan?

Answer: 
There actually is, the VOY episode Flashback, as someone mentions here, retcons Tim Russ's human appearance during the original series movies and explains that Tuvok was in fact an ensign/lieutenant during that period of time --- Tuvok is show to have served on the Excelsior during Undiscovered Country as an Ensign and specifically mentions later serving with/meeting Kirk and Spock.
Although this isn't an official piece, my favourite personal theory for Tim Russ' appearance elsewhere in Star Trek is because he's an undercover agent (he had previously infiltrated the Marquis and had been brainwashed during said stint in the official VOY diaglogue). Make-up and physical changes aren't uncommon in Star Trek - such as when Janeway becomes borgified.

So it might well be, as a Vulcan with a 200 year long lifespan, that he could very well appear both on Enterprise-B, then later a mercenary on the Enterprise (especially as this might have been during his undercover stint for the Marquis - we never actually see him die, he could have just cleverly beamed out back to his ship whilst screaming to fool Picard) and then again as the Klingon mercenary on DS9 (perhaps trying to expose any Dominion connections?).

This might explain, after all, why Tuvok is such a badass.

Who knows? But it's the theory I like to entertain. Makes it more interesting.