I feel as though if you have the costume, but not the personality, then you really shouldnt cosplay at all.
What fun is dressing up if you arent going to act a bit? In my case, i have a Maes Hughes outfit. Why? Because i look like him AND act like him as well.
Also, be sure that you are going to use the costume at some point in your life. Be it for random fun, or for some even with friends, don;t buy the outfit just because. Always have it for some use.
A quote from Alien versus Predator really brings my point home: "Same principal as a condom. I'd rather have it and not need it, than need it and not have it"