Fill-up or Fill-out

Hi,

Actually, it's a slightly odd case where you can fill out a form, or fill it in. Both are OK. But you can't say to someone 'Please fill up this form', it's just wrong.

Best wishes, Clive

Well, native English speakers would never say 'fill up' a form. It may have become a local variant perhaps, if you hear it a lot, or it could be something that people bring in as a direct translation from their native language without considering that the preposition may be different in English.

In mainstream English either 'fill out' or 'fill in'

Thanks Nona! Does it soudn weird to you because fill UP is usually associated with containers? For example, fill up the bucket, fill up your glass, fill up your tank/car. I would like to know how native speakers interpret the preposition 'in' in this phrasal verb. Thanks in advanve

Edit

Tanit, thanks

My question to you: .Clive said something about 'odd case' and I can't relate that to the rest of his sentence. To me, odd is negative but later he said, both are OK! I detect contradiction :(

I have no problem with Fill in and that's what i hear and use a lot of times. I think I must have misheard out for up. A short sound like this is easily mistaken by me especially the speaker is talking like a bullet train. 

Also, fill OUT has a sense of thoroughness that fill IN doesn't have. Am I right?