Just curious, as an SIC at a 121 carrier, how would you perform the duties of the PIC? PIC in this case is not manipulator of controls, it is exerting final authority for flight. How do you act as PIC in this case? Can you overide the CA? Also, don't most FOM's say that the Captain cannot delegate his PIC responsibilities to anyone else. I know ours does. For example, It is the duty of the PIC to sign the release, can you do this as an SIC? Can you sign the aircraft acceptance check? Can you demand more fuel from dx on your own authority? What PIC duties exactly are you performing with supervision?