Do you fill out a form or do you fill in a form?

These two phrasal verbs are interchangeable, but fill out seems to be more common in American English and fill in in British English.

If there is a difference, some speakers maintain that you fill out a form or document and fill in the blanks. In other words, when you complete a form, you fill it out, and when you add information to empty fields, you fill it in.

Fill in is also used to mean to give somebody missing information.
My boss filled me in about the latest development.

Fill out also refers to gaining weight.
You’ve filled out since we last met.