Radical Acceptance says that life works better when you accept people for who they are, without judgment. Once you have done that you can then act accordingly.
What does that mean? Here is an example. In a car the accelerator pedal is on the right and the brake is to the left. That is not a big deal. I don't see impassioned pleas to rearrange the pedals. People know this and accept it without judgment. Now let's imagine that you think having this arrangement is bad. Furthermore, since you don't like it, you are going to act as if the pedals are reversed. It won't take you long to come to grief.
But failure to accept people is like the example above. How many of us see, not the person in front of us, but the person we expect to see. Then we act as if they are how we expect to see them. And, sure enough, we come to grief.
Come to know them through their actions, accept without judgment and act accordingly. If the other is a controlling person, then I know I will either need to let go my need for control or be prepared for a battle of wills. If the other is chronically late, then need to invite them early to events, be prepared to start without them or wait. In any case, to get upset is to pretend they are someone different - and that's like pretending the accelerator and brake are reversed, a quick road to grief.