"Got on a soapbox" is an idiomatic expression that means to deliver a speech or a long-winded rant about a particular topic in a passionate and sometimes aggressive manner. Antonyms of this phrase may include being reserved, unassuming, or silent. Being quiet, modest, or passive may be seen as opposite behaviors to someone who has "got on a soapbox." Additionally, being empathetic, understanding, or diplomatic could be opposite actions when someone is using a soapbox to push their own agendas without considering perspectives from others. Overall, the antonyms of this phrase suggest restraint and consideration, rather than outspokenness and aggression.