Interesting topic as I have heard many FO's reflect on Captains that "give duel" while they are flying. I'm not talking about situations that envolve safety or variation from SOP's, but technique.
As a fairly experienced MD-80 instructor and Check Airman, I have given many Line Checks and watched crew interaction. In my experience, when a Captain continually corrects an FO, due to technique, it is usually because the FO is doing something the Captain isn't use to or hasn't seen. Possibly even flying better, especially when it's "hands on, autopilot off". It usually comes from lack of experience or lack of confidence on the Captains part.
If the FO is operating in a "relm" the Captain is not use to, the Captains confidence may be envolved. There is more than one pilot/Captain out there that shouldn't be in the position. I believe the definition of the PIC is "the master of the aircraft". Lowering the flaps or Landing Gear at a different time/position, say on a Visual Approach, is no reason to "prompt" or "pimp" the SIC.
Everyone doesn't fly the airplane the same and every situation can be different. As long as safety and SOP's are adhered to the FO should be left alone to fly "his leg" and his skills respected.