you must assume he/she has "lost it" and take over the controls.
It is not quite as easy as that. It is one thing to calmly take over control if the captain is incapacitated. An inexperienced F/O may run into trouble if he lacks the initiative and skill to rescue the aircraft. Although nominally second in command, many recently graduated cadet first officers lack the ability to become the instant captain with no one to guide them.
It is an entirely different situation if the captain is reckless and intent on breaking the rules of good airmanship and in so doing risking lives. While there is no shortage of well-meaning advice on what to say to that sort of captain, I have yet to see in company standard operating procedures any firm directions on the physical method to be employed by a first officer if the captain refuses to listen to reason.
If the problem occurs on final approach to land and the captain is the press on regardless type of personality, then an unstable approach is one result. Short of hitting the offending captain over the head as a precursor to taking control, the other thing that is easier and less dramatic is for the first officer to simply select the landing gear lever to up at the same time saying aloud "GO AROUND CAPTAIN" That fixes the problem real quick until the next approach of course when tempers will by now be frayed. But cross that bridge when you come to it
