It's perfectly normal to be a bit nervous or shy during interviews, so don't sweat it so much. A decent interviewer isn't going to hold that against you. I'm more suspicious of people who aren't a bit nervous, and am inclined to interview them more thoroughly because most of the time when companies I've worked for have hired someone who's slick, they've turned out to be an imposter.
An interviewer held it against me. Said I did well technically but didn't seem confident enough.
In order to ace the interview you pretty much have to be the perfect candidate -- technically, socially, emotionally.
As he was completing the coding portion of his interview, I broached the topic of comments. "Nah, this is an interview, it doesn't matter."
This was back in 98 and he had a little personal website with a discussion board for his friends. He whined there about not getting the job, so I explained that it was his attitude, and not his skills, that made him a bad fit. His friends seemed to agree with me, that getting taken down a notch was a necessary step in his emotional growth.