Step In' is a term that can be used to describe a variety of actions. INTERPOSE, INTERCEDE, INTERVENE, INTERMEDIATE, ENTER, and Replacesomeone are all words that can be used to describe the action of 'stepping in'. INTERPOSE means to insert oneself into a situation or conversation, INTERCEDE means to INTERVENE on behalf of another, INTERVENE means to come between two people or groups, INTERMEDIATE means to act as a mediator, ENTER means to come or go into a place, and Replacesomeone means to take the place of someone else.