I always figured that he was, like Boba Fett, somebody simply who wasn't afraid to talk straight to Vader. Fett clearly has no power or authority over Vader, but he showed no fear of him either. Tarkin is much the same way. He has his turf, as does Vader. He's simply not afraid to let Vader know when he's going a little too far. And I think Vader respected those who had the balls to stand up to him but weren't idiots. Ozzel was an idiot and was killed for it. Same thing for Needa. Of course, they were responsible for Vader not getting his son back, not just regular back-talk.

By the way, in ANH, Tarkin is the Governor of the Outer Rim territories. I thought Motti was actually in charge of Death Star construction. Or was that Tagge? I get them mixed up.

Anyway, I do agree that the story could have benefitted more from additional exposition of Tarkin. I noticed the dialogue about the deleted scene where Palpatine gave more authority to the Governors and thought this directly addressed the Tarkin situation. Of course, Lucas is notorious for cutting info that advances plot and character development. But we got buzz droids! Lots and lots of buzz droids.