Is it normal to act a different way at work?

Then in your personal life. Well,I work at Sears and I've notice this in myself as well but for some reason I don't even know why I do it. When I'm not at work I'm outgoing,joyful etc etc. When I'm at work I'm very quiet,monotone. What are some types of ways I can get over this? I'm glad one of my co-workers told me because I would really love to work on this. You have to sell those credit apps and no-one will want to buy it from a person who doesn't even sound like they are enthused about their companies own product. Is this like a case of stage freight or not really?