Just think of all the people before and after you that are also going to be interviewed for the same position. If all other things were equal - qualifications and the answers to the interview questions - what is going to set you apart from the rest? You can be enthusiastic and smile when answering (when appropriate) and still maintain an aura of professionalism. You want to exude charisma and keep the interviewer's attention. They have heard a lot of the answers already, but you can get the message across with more than words.
Someone who is excited to get a job and lets that excitement be known is going to have a better chance than someone who talks in a monotone and with little to no emotion. Don't be afraid to smile and use phrases as "that's great" or "wonderful" when you are told about the company. Be the type of person that the company wants to represent them and you will increase the chances of a job offer.
A few words of caution: don't go overboard. Be genuine in your enthusiasm and be yourself. Sincerity is key or your enthusiasm could work against you instead of for you. If you are naturally bubbly by nature, tone it down a bit for the interview so you do not overwhelm your hosts.