informal have as much of something as one wants; do something to the full:fill your boots with spicy Szechuan food for under five bucks a plate
fill someone's shoes (or boots)
informal take over someone’s function or duties and fulfil them satisfactorily.
fill in
act as a substitute for someone when they are unable to do their job:my producer will have to have someone standing by to fill in for me
fill someone in
1inform someone more fully of a matter:they filled me in on all the latest news from Cambridge
2British informal, dated hit or punch someone:I filled in a chap and took his money
fill something in
1chiefly British add information to complete a form or other official document:simply fill in the application form and return it to your local branch
2complete a drawing by adding colour or shade to the spaces within an outline:incised letters, filled in with gold
3occupy one’s spare time, typically while waiting for something else to happen:with all the shops to keep you occupied, you’ll have no problem filling in a couple of hours
fill out
(of a person) put on weight to a noticeable extent.
fill something out
chiefly North American
add information to complete an official form or document.
give more details about something:further research will fill out these early findings