The antonyms for 'act up' include words like behave, conform, obey, cooperate, and comply. These terms suggest that an individual is following the rules, behaving in a well-mannered way, and not acting out of line. When someone is behaving, they are considered to be in control of their emotions and actions, while obedience and compliance suggest a willingness to follow the commands of others. In contrast, acting up indicates refusal to comply with rules, regulations, or expectations. These antonyms offer a useful way to communicate positive behavior and can be useful for parenting, teaching, and other roles requiring discipline and structure.