All i can say is, if the hall contest you entered had a panel of judges that just sat there and stared at you, no wonder you got nervous.
Good judges, be they hall judges or masquerade judges, are the ones asking the questions. I like to start with a generic "so tell me about how you made this," or "what do you want me to know about how you made this?" and then ask leading questions from there. We know people can be nervous and might not know what to say, especially if they're novices, so the more questions we can ask them, the better. Then they start to feel comfortable about talking and may eventually volunteer information for us.
I still compete at international level, and at some regional cons, so I know what it feels like from both sides. I honestly prefer the hall contests that are just some roaming judge who thinks your costume is beautiful handing you a ribbon - that way the only panel interview I have to do all weekend is for the masquerade itself. That can be nerve-wracking enough even for a master! I don't like adding onto the costume stress by making hall cosplays go through the same thing. At that rate, just enter the stage masquerade, really.
BUT...that said. The judges shouldn't be unhappy if you bring them reference materials and maybe some notes about your costume, even if you just read them the notes. At home while you're working on the costume is where you remember all the important things to tell them, about what you learned, what you're proudest of, etc, but all that goes out the window at the con. So go ahead and write it down, and explain it to them off a notecard if they don't ask enough questions or give you a comfortable space in which to show and tell your costume.
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com