The phrase "in the act of" is commonly used to describe someone doing something in the moment. However, if you're looking for other ways to say it, you can use synonyms like "in the process of," "in the midst of," "engaged in," or "actively performing." These phrases all convey the same meaning as "in the act of," but provide variety in your writing or speech. For example, instead of saying "He was caught in the act of stealing," you could say "He was actively performing theft when he was caught." Using synonyms for common phrases can make your language more interesting and engaging for your audience.