We act out when we fail to handle our emotions. We shout or we throw tantrums.
Showing reaction on others is nothing but the externalization of problem. There will be something hidden in the problem, that is directed towards us. If we figure out what it is and try to resolve it, acting out stops. When acting out stops, we dont react, we act!
Example, imagine a girl shouting at her friend for delaying their group project submission. External version of problem is, project submission is being delayed because of friend. Internal version is, girl cannot tolerate delayed submission, most probably because she linked timely submission with her happyness, self esteem and many more things.
(there are other articles about linking of things in detail)
She has right to submit on time. She need not compromise or delay. But unlinking of her values with submission is important to stop reacting. i.e, stop shouting. Shouting cannot help her. Preventing reaction helps her to act. Acting involves doing something that helps her to actually submit the project!