bkev said:
*waits for Anchorhead to answer all questions*

They work in the labor pool of the Tantive and are picked to be used to receive the transmission of Death Star plans while appearing to make repairs to the hull. They get caught by the Devastator, the Tantive flees and that's where the film starts.

canofhumdingers said:
I always assumed that the droids belonged to the captain of the Tantive IV ("our last master was Capt. Antilles...")

R2 is instructed by Antilles to listen to Leia's voice for recognition and follow only her orders. That's why he, for the most part, ignores 3PO and "babbles on about his mission" and continues on it whenever he gets a chance. He's under her direct orders, not some sort of loyalty.