There are two seats up front for a good reason and it is not a master/slave relationship. Some of the comments above hark back to the bad old days where two-crew aircraft were run as single pilot ops. Remember, plenty of meek, obedient FOs have sat there in silence and allowed the Captain to fly them into a smoking crater. You have a responsibility to speak up to ensure a safe and satisfactory working environment. You have a responsibility for your ass, your life and the safety of the operation.
Despite what some may think, just like Popes, captains are not infallible, and the spectrum spreads from likeable, competent operators, to guys who are a bit difficult to work alongside, to those who are a downright threat to the operation. In the latter case if you as FO do not challenge such threats you become culpable, practically and legally.
As FO you have a veto in that the flight isn't going without your cooperation. You better be damn sure before you use it or even threaten to use it, but it is there. Only once have I ever had cause to challenge the guy in the left-hand seat, and that was after he caused two separate reportable events, one with potential for injury or death, before we even started taxying. I was genuinely concerned about the prospect of getting airborne with this captain. The only negative feedback I got from management was that I should have stopped him sooner (although in reality that was not practical). If I had allowed this pilot to continue the flight the way he was going anything could have happened and I would have been blameworthy for facilitating it. In these cases the CVR is your friend. Make sure you are squeaky clean and that it's all on record.
Incidentally, although I am an FO in my current job I have been Captain on multicrew aircraft elsewhere. You are both there to look out for each other and that doesn't mean always saying yes to the other bloke.
Last edited by Torque Tonight; 26th February 2013 at 11:38.