Are the expressions COVER FOR SOMEONE and COVER UP FOR SOMEONE freely interchangeable in both meanings or only in one:
cover for someone
cover up for someone
cover for someone - 1. carry out the essential duties of a fellow employee, absent through sickness, etc.: I will cover for you while you are away.
2. conceal smb.'s wrongdoing; provide an excuse for a person: She said that Zaikas had covered for Isaac, who was a party to the state-hospital scandal.