Casualism is a term used to describe a style that is relaxed, easygoing, and informal. The antonyms for this word would be words that describe a style that is more formal, structured, and rigid. Examples of antonyms for casualism include formality, strictness, seriousness, formality, and severity. These words describe a style that is more focused on rules, regulations, and expectations. It is also possible to use antonyms like rigidity, harshness, tightness, and severity to describe a style that is more uptight, uncomfortable, and unforgiving. Whatever the antonym used, the opposite of casualism is a style that is more formal, rigid, and structured, and leaves little room for improvisation and playfulness.