How do convincing speakers vary their speech when they speak for different purposes and to different people?

it depends on who are the people they are talking too, and it's all about changing the tone, if talking to a customer they try to give them the best offer and have a relaxed, fast tone, they talk about all the advantiges of the products and the saving that can happen. 
if talking to a friend or a relative or family member, then their speech will be relaxed, usual talk, no nervous or worrying about the come back.