The phrase "acts a part" means to behave in a manner that is not true to one's nature or true intentions. Antonym for "acts a part" would be to "behave genuinely," that is, to behave in a way that aligns with one's authentic self or true intentions. Other antonyms for "acts a part" can be "sincere," "authentic," "spontaneous," "honest," "natural," or "forthright." These antonyms emphasize the importance of being true to oneself, behaving in a way that reflects one's sincerity and genuine intentions. In contrast, engaging in behaviors that are out of sync with one's nature can lead to feelings of discomfort or a sense of inauthenticity.