I know of a place or two who got caught picking people using beauty as a part of the criterion. A clothing store in the mall was one. However, I suspect that few jobs will do that.
As for screwing up interviews, I cried in the middle of an interview because I thought that I would never find a job. I am looking for work again and pray that I willl never meet that guy again. So, if you do screw up the interview, remember that others have gone before you and screwed it up too. He/she will know that you are nervous.
I am not sure how to fake confidence. My T said that line about faking it until you get it. I didn't care much for that line but it is the same as saying that one has to practice any knew behaviors to get good at them. I would say that some places allow you to practice interviews. Workforce development in my area has a program of practice interviews. Then you get advice on what you can do better on. I am to chicken to do it because I think that if I am going to make a fool out of myself, I might as well do it while trying to get a job.