For all intents and purposes, the first officer, no matter his experience level, becomes the PF or captain and is encouraged to make all the command decisions while the real captain sits somewhat uneasily in his new role as PNF or co-pilot
That is not how it is in any of the airlines I have flown in. The F/O becomes the PF but doesn't magically morph into the Captain. For example, if the F/O is PF and wants to cruise at a higher level than they currently are,he or she would probably say something like " I'd like to climb to 380, are you happy with that?" and the Captain responds with whatever they want.
Sometimes it is more appropriate for the f/o to fly a non precision in poor weather, the FSF lists Captain as PF as a risk factor for CFIT, it's pretty obvious why.