There are several synonyms for the phrase "step in" that can be used to convey similar meanings. One possible synonym is "intervene," which refers to taking action to affect a situation. Another option is "intercede," which means to intervene on behalf of someone else. "Mediate" could also work, as it suggests acting as a neutral third party to help resolve a conflict. Other related terms include "intrude," meaning to insert oneself into a situation uninvited, and "butt in," which denotes a more abrupt or rude form of intervention. Overall, there are many ways to express the idea of stepping in or taking action in a given situation.