Sometimes talking about your costume becomes kind of like a class presentation.
I know from personal experience that my level of talkativity usually depends on the vibes I get from the judges - sometimes, in the case of judges that appear to be bored or distracted, I get so tripped up that I don't explain myself very well - even if I have an entire speech memorized and know what I want to highlight, I go blank! The index cards are a good idea but I don't advise anyone to rely on them too heavily - use them as a guideline in case you feel nervous, and for cosplayers new to competition, it can really help with memorization and confidence, I think.