Never had to. Challenge an issue once, wait for the correction. State it again more urgently, wait for the reaction. "My controls" after you've stated the issue twice. This will all happen very quickly if it ever does.

If the safety of the flight is in immediate jeopardy you might have to take the controls without challenging. In fact I brief guys I haven't flown with before that I fully expect that from them. That road goes both ways. That's why there's two of us. Work together to keep each other safe.

Luckily I've never had it happen either way.

If you truly are afraid to fly with the guy/gal you need to voice your concerns to him/her. If they don't listen and/or blow you off go directly to your CP and don't fly another leg with them.

You have a specific issue you wanted to run by us? If you feel better to PM feel free.