fill in

Usage

British English. American English prefers 'fill out'; both are understood everywhere.

Separable

The object can go between the verb and the particle: turn the light off, or turn off the light. When the object is a pronoun it must go in the middle: turn it off, never turn off it.

Practise it

Add it to your daily practice or see every verb with up.

Further reading: fill in on Wiktionary (CC BY-SA).