The_Tempest
Senior Member
Per the original back story, the Emperor was more or less a Nixonian figurehead whose practical authority was usurped by underlings like Tarkin. Originally, Vader nominally deferred to the Emperor but was also subordinate to Tarkin, who really ran the show.
Now, of course, we know the Emperor was manipulating Tarkin and Vader the whole time. With the release of all 6 films and the EU, things were retconned. Tarkin and Vader don't particularly like one another but each respects the other. Between that and Tarkin's position of authority on the Death Star, he felt comfortable barking an order or two at Vader.
Also, Vader's placidity is one of the best things about him in TFU2. Dude had no fear, whatsoever, even when outmatched.