There's that ol' idiom that proves to never be entirely true, “There are two types of people: those who (blank) and those who (blank).” As deceptive as I've always found these statements to be because of their sweeping generalizations, the more I think about it, there's a lot of them that apply to Halloween costumes.
I personally have pretty much always thrown something together last minute, soon after finding out there's a costume party. Then there are those who premeditate on their costume well in advance, with the same kind of eager anticipation you find in people who start listening to Christmas music as soon as Halloween is over. I'm not even sure what would be the standard amount of time one should set aside to plan their costume, but I've never known anyone who started thinking about it at a time that seemed reasonable.
Usually, people go for one of two approaches. The first, being dressed in something that doesn't use a lot of material and makes the opposite gender drool, the second is a costume that makes an allusion to something that person would consider part of their identity, i.e. the primary purpose is to express themselves. Then there are those who go for the most recognizable and relatable “I don't want to spend all night being asked who I am” type of costumes versus the incredibly niche reference that's so obscure not a single person guesses it, and the wearer goes home that night and sleeps soundly knowing how unique they are.
If you want to be judgmental about it, you probably could tell a lot about a person by the costume they picked. Whatever costume you decide to wear for Halloween this year, just be yourself! Or, uh.. yourself being not yourself. Does that make sense?