fill in /fill out

English is a nightmare because of the use of such adverbs and the use of prepostionspropositions (in, out, etc. are adverbs; to, from, etc., are prepostions). Many of the usages--i venture to say most of them--are only to be learned in their coloquial usages, as opposed to following rules. Upon a time, generally speaking, Americans filled out forms, while the English and others in the English speaking world filled them in. I find that these days, fill out or fill in can be used interchangeably. Either is correct and will be immediately understood by the native speaker of English.