Never, ever, ever judge a book by its cover. Don't do it because it will come back and bite you in the backside.
I know that I have done it many times where I have judged someone by the clothes they are wearing or their hairstyle or even by how tall they are.
This is just not a good way to look at life and other people. I'll give you an example. A guy was setting up a room I was in the other day. He would have been about 4 foot 11 and had weird hair, kind of a comb over and he had funny teeth. Of course I passed judgement and it was all wrong. He was fluent in English, ultra-intelligent and he loved Chopin. I played something on youtube and he immediately recognised the music. I honestly felt very embarrassed.
Whenever you meet someone quiet don't think that they don't have energy. Don't think that they are not very intelligent because they don't say much. If you underestimate an introvert you are going to come out as the loser
Don't judge a book by it's cover. The introvert might not be saying very much but they have a lot of good ideas under the surface and they will be out hustling you before you know it.
Just because he or she is not talking, is not being exuberant does not mean they can't do anything. Underestimate them at your peril.