In the original Star Wars trilogy we can see from episode IV to VI the respect shown by Darth Vader to his Emperor, Darth Sidious even when the latter was not present.
However, I find a specific scene quite curious. In Episode V, we see for the first time the face of the Emperor through a holographic projection. Vader bowed, his head down, he raised himself while keeping one of his his knee on the ground and said :
What is thy bidding, my master ?
Looking up the use of the archaic English, we can see that "thy" comes from the singular informal of the 2nd person
"Thy" was - and is sometimes - used to address someone we see as an equal or that we hold affection for. But that word can also be used toward someone we disrespect, disregarding his rank or position.
I don't think Vader held much love for Sidious even if he was the last person Vader considered somewhat as a close ally. Seeing the decorum Vader showed to his Emperor in episode V and VI, I don't think the both of them viewed themselves as equal, Sidious was clearly above him.
So it raises a single question :
Why would Darth Vader use that form of speech toward the only man he was supposed to respect ?