"Step in for" is a common phrase that means to take over or replace someone in a particular task, role or situation. There are several synonyms available for this phrase that can be used interchangeably, such as substitute, cover, fill in, replace, stand in, deputize or take over. All of these words suggest the idea of someone temporarily performing a task or job in the place of someone else, providing necessary support or relieving them of their duties. These synonyms are useful as they provide variety in writing and speech, allowing the speaker to convey a varying degree of seriousness and urgency when needed.