Eye contact is paramount when you are talking to someone as it shows that you are listening to them and are acknowledging what they are saying. If you are looking away or doing something else then this can come across as you are not interested and the individual may stop talking to you. Also if you have your arms closed this can seem like you are angry and people may not want to approach you with a problem. The tone of voice is also important as this can show how you are feeling and you can emphasise words for effect.
Speaking loudly may come across as you being aggressive which may scare some people or cause conflict. Some may feel intimidated by you and may wish to not work with you or make a complaint. However, if you are a quiet talker then you may come across as shy and people may feel comfortable trying to speak to you. It is also useful to have a clear and cheerful tone as this will ensure that the information you are giving is correct and others feel comfortable being around you.