A character's style of speaking is an extension of the character's personality. A character's vocabulary is effected by by the vocabulary that the character has been exposed too (vocabulary in the home when the character was growing up, books that the character has read, people that the character has interacted with often... ). A character's etiquette and demeanor is also influenced by all of the above, and training (self-taught or professional training), because it also involves body language.
Originally Posted by zizi
Think about a character's personality. Does the character like to get to the point? The character's dialog may be short, and to the point. Is the character impatient? How does the character deal with something that tests its patience? How would the character speak? Does the character have weak self-confidence? would the character be apologetic because it sees it sees itself as being at fault?
Using a character's dialog as an extension of the character's personality is good step to take first. Think about the reasons behind the character's actions, the personality that is displayed by those actions, and how that personality would be portrayed through the character's words.