I have always been puzzled and confused of these two phrases: "Fill-up" and "Fill-out". Whenever I go to the Professional Regulations Commission (PRC), I am always instructed to FILL-UP the forms. It even has a big instructions posted on the wall stating to FILL-UP the forms completely. I even hear professionals use the phrase. I always see such also in many of the forms from both private and public entities. But, a teacher of mine told me to use FILL-OUT instead. Why should the preposition be changed?