1) The backstory of each character
3) Form of speech (according to education level)
4) How they are perceived by others (first impressions)
6) Character conflicts
Since we have previously taught about the role of backstory and MLR we won't go there. Number 3 on our list is a very important tool to use while forming your characters. Afterall, you don't want your characters to all sound alike. The trick? Pay attention to your characters education level and backstory. Do they make use of pet phrases? Is there speech always informal? Do they tend to use more slang during times of stress or become more formal in their speech at those times?
What you don't want is two characters that sound alike. If possible, you can establish speech patterns for each character to such a degree that even without reader tags (he said, she said), the reader knows simply by the way the character is talking who is speaking. Think of Yoda from Star Wars. His speech was distinctive based on the kind of creature he was, and when reading the book, you would automatically know, just by Yoda's speech pattern, to whom the dialogue belonged. Does each character require that degree of variation? No. A few well-placed pet phrases or commonly used slang words can fit the bill.