Robert's backstory. In the film prot's complete breakdown as Mark forces him to relive the murder of his family and his killing of their murderer before trying to kill himself.
prot reflecting as he prepares to leave that he is never missed when he leaves K-PAX, but he senses he will be missed when he leaves Earth. Mark all but begs him to stay, clearly not just as a shrink worried what he'll do when the time comes but as a friend who doesn't want to see him go. prot slowly responds a moment later with "I will miss you, Dr. Powell."