Depends on the state. In Pennsylvania you have the same right to use force to defend a third party as you would if you were the one attacked.
What they drummed into us in CCW class, though, is that we had better be DAMN sure that we knew who the good guy and who the bad guy was in any situation where we intervened. If we shot an undercover cop, we were screwed.
In most cases, though, that's clear enough. If there's a guy holding a knife to a woman's throat, he gets a bullet.
I’ve not had any situation requiring any force other than words since I was a kid. All I can do is give it thought but I imagine you could tell what’s going on.
I hope in never happens because it would be a huge major league hassle to use deadly force. My thinking is once the weapon comes out it WILL be used.