You would obviously say something to the guy, see what his thoughts are about what he is doing, if you can get an understanding then you can make an informed decision about what you are going to do at the time, if you deem it to be unsafe, tell him, see what his reaction is, I as a captain would have no problem if my FO questioned what I was doing especially if it was not standard. Taking control straight away before exploring the issues is quite an extreme action, however it depends on the phase of flight and how much time you have to act. If in doubt speak to a Captain you trust see what he thinks, take his advice, speak to a trainer if necessary. Hope this helps