Tough question. The Captain's authority is final, right up to the point where he tries to kill me. At that point, I will bash his skull in with the fire extinguisher and save myself. If it's just a matter of breaking SOP or regs, I will generally stick with vocal interference. If I'm in command, I will frequently say something along the lines of "Don't let me do anything stupid", and if an FO does call me out on an altitude bust or some such, I will be sure to thank him.