Use 'fill in' for blanks or spaces on documents.

Context Specific

'Fill out' can be more common in American English than 'fill in'.

Beware Of Overuse

Commonly used in casual conversation to request or give an update.

When filling someone in, give essential information without overwhelming details.

Emphasizes temporariness when substituting for someone.

Does not usually imply a permanent role change.

Related to making an empty space solid or complete.

Suggests repairing or fixing something that is incomplete or damaged.