fill in
(to take the place of someone who is absent)
- Jane is on maternity leave, so she asked me to fill in.
- I’m trying to find someone to fill in for you.
fill someone in rather informal
(to provide information)
- Could you fill me in on what happened yesterday?
- Jim was absent from the meeting. He needs someone to fill him in.
- Why don’t you ask John? He can fill you in on the details.