At the beginning of A New Hope, Threepio knows about the Princess. Then when he is on Tatooine and Luke asks who she is, Threepio doesn't know. Did he do this on purpose?

C-3PO was prevaricating as he was programmed to do so. This is elaborated upon in the radio dramatization, in a scene prior to the attack on the Tantive IV. Threepio's master, Captain Antilles, enacts a voice override command protocol, allowing Threepio to disregard some of his ethical programming to carry out important tasks. Here's what Antilles says to both C-3PO and R2-D2:

"This is a command/control instruction. Both of you will restrict and protect all references to Leia Organa's identity and presence inboard this vessel. She is designated a command/control voice."

Posted: by Jedi Power