Mark's not at work today, who will stand in for him.
Mark's not at work today, who will fill in for him.
--- Do they mean the same thing?
So it's better to say "fill in for" in this case?
So when would you use "stand in for"?
[man who is to give a big presentation to a group of investors phones in sick]
Man: Hello, Boss. I'm in the hospital with a case of appendicitis. I'll be going into surgery after lunch.
Boss: What?! I guess we'll have to cancel the big meeting.
Man: No, don't do that. Brian has agreed to stand in for me. He knows this subject as well as I do. He's been working on this thing with me for a month and I know that he's a better speaker so things should go perfectly.
Boss: Well, if you say so, I'll have to trust you.
stand in for - idiom
Substitute for, as in He's kindly agreed to stand in for me at the reception. [Early 1900s]
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
stand in for - meaning