1. Someone who takes the place of another (as when things get dangerous or difficult)
   "we need extra employees for summer fill-ins";
   - stand-in, substitute, relief, reliever, backup

1. Write all the required information onto a form
   "fill in this questionnaire, please!";
   - complete, fill out, make out
    
2. Supply with information on a specific topic
   "The manager filled in the new employee on company policies"
    
3. Be a substitute
   "The young teacher had to fill in for the sick colleague";
   - substitute, sub [informal], stand in
    
4. Represent the effect of shade or shadow on
   "She filled in the darker areas of the landscape";
   - shade

Derived forms: fill-ins, filling in, fills in, filled in

Type of: change, compeer, draw, equal, exchange, inform, interchange, match, paint, peer, write

Encyclopedia: Fill-in