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human
[Star Wars] Why is Grand Moff Tarkin not afraid of Vader?

We see on screen that when he commands Vader to let go of the force choke, he asserts a sort of sternness. This makes me think Vader was lower than Tarkin in the chain of command. Does this mean even in a room with each other, Tarkin was fearless of Vader? Or was it just a meeting room thing?
gpt
Tarkin and Vader are longstanding colleagues and have worked together since even before the end of the Clone War. They're almost friends, and essentially equal in the chain of command, both second only to the Emperor.

Tarkin is also one of very few people who know the true identity of the Dark Lord.