I was told by someone at an employment agency that it's good to show some nervousness and eagerness because it's a sign that you really care about getting the job and are eager to work.

I've never gotten a retail job but I've been on some interviews for retail. It's odd. The employers seem to have fun playing with minds and psyching people out, testing their customary skills and character. You have to be able to self-promote and know the products and trends ahead of the interview and know what you're talking about. There is little room for training and gaining knowledge after being hired, so they want someone who's ready to sell stuff right then and there.
I've blown so many retail prospects. My weakness is saying goodbye and thank you. I don't know how many times or how big a thank you I should say. If I should initiate shaking hands or wait for them.
I'm horrible at math and often they want someone who can count change without automatic tills or calculators. I've tried to "sell" what I was wearing with little knowledge about the exact name of the style of clothes I was wearing but put on a more fashion show-esque way of doing it. I don't know if the managers meant "that was different" in a good way or bad way. At least I have a personality.
One job I was trying to go for an interview at, the manager grabbed the phone and started a conversation just as I stepped into the room. I waited patiently for him to finish. Then he told me that the interview was done. Oookay. Turns out, a neighbor down the street that worked there and put in a word for me to get a job there, said the manager wanted me to interrupt his phone call and that would have shown I was aggressive. That type of aggression is not me and so I'm glad I didn't get that job.