I usually use the form "fill out" in sentences like "fill out the questionnaire, form, survey, etc." However, the other day, I said "fill up" instead of "fill out". I was very tired - it was a slip of the tongue. Nevertheless, it got me thinking: is it technically a mistake to say "fill up a form" instead of "fill out a form"? Just because we're more used to one form, does it automatically make the other incorrect? Thanks in advance.

You can fill out a questionnaire.

You can fill in OR fill out a form.

You can even fill up the entire space alloted on a form for a particular question.

But don't fill up a form or fill up a questionnaire.
Can you fill in a questionnaire?