What is the difference between (if any) "fill in for" and "sit in for"? Are they used interchangeably ? If yes, then what is more likely to be used?
Like a teacher is ill so another teacher comes to teach the class for a day:
Your teacher is ill, so I'll be filling in/sitting in for him today.
(The teacher may or may not sit)
The Manager is busy, so another person had to attend the meeting.
The Manger is busy, so I'm filling in/sitting in for him today.
(And all sorts of such contexts......) Thank you:)