I had this exact issue come up once with a Capt while flying as a new hire on a T Prop airliner. I found it extremely disconcerting and talked to the Captain who replied that he always "helped" the FO's on landing and was not going to change how he ran "his" airplane. I then raised the issue with the CP who was surprised and shocked at what was going on. The result was a memorandum that gave clear direction that Capt's were not to make any control inputs without first making the "I have control call". The Captain in question eventually left the company as he was about to be demoted for general incompetence.
Bottom line is you are either PF or PNF , there are no SOP's for "Pilot flying a little bit". This practice is completely unacceptable and as was noted can lead to unexpected aircraft reactions, particularly inn FBW control systems.
411A , I am betting you never shadow the controls as you have enough skills and experience to know how far to let an FO go and when the line is crossed just take full control. If when acting as a TRE you observed a Captain "help" an FO with unannounced control inputs, what would you do ?