Acting is the action you are supposed to do, at that moment. When you cultivate yourself, that is action. Reaction is that you are influenced by the situation and you are confused by the situation. We give ourselves into the situation where we cannot act. Then we react.
Reaction means that the power of certain circumstances comes into our mind.
If I am mad at you, if you act, you won't get mad back at me. If you react, you will shout exactly the same way because you have taken my anger. You will answer with the same manner or twice as angry. When you act, you keep yourself calm and centered and do not get affected by my anger. A situation for a person who has not cultivated him or herself makes them angry or agitated.