The term 'fill in (for)' can refer to a variety of different activities. A TEMP is a temporary worker, usually hired for a short period of time to fill in for an absent employee or to cover a specific job. A SUB is a substitute, someone who takes the place of another person for a certain period of time. SPELL refers to spelling, which is the process of forming words with letters in the correct order. To RELIEVE is to take over for someone else, often temporarily, in order to provide relief from a task or responsibility. An ACT is an action or deed, and actas is an abbreviation of the phrase 'ACT as, ' meaning to take on the role or responsibilities of someone else.