What is it called when you fill in for someone?

When you fill in for someone, it is called substituting, standing in, covering, or subbing, and the person doing it is called a substitute, stand-in, replacement, backup, relief, temp, or cover. In workplace contexts, it is called covering a shift, providing coverage, or working as a temporary replacement. In entertainment, it is called being an understudy, stand-in, or pinch hitter. In sports, it is called being a substitute or bench player stepping in. The action itself can be described as stepping in for someone, taking over for someone, holding down the fort, or holding the place. In formal terms, it may be called acting in a capacity, serving as interim, or assuming duties. For example, if a teacher is absent, another teacher fills in and is called a substitute teacher. If a colleague is on leave, you are covering their role. The phrase fill in for someone emphasizes temporary nature, implying you will return the role when original person returns. It is common in professional, academic, and casual settings to ensure continuity.