style - How to create varying, yet realistic, speech patterns

Different characters speak differently. If someone reads me a letter written by an immediate relative, I can tell right away who wrote it. I believe --and correct me if I'm wrong-- that giving each major character a subtly unique way of speaking can help make the character seem more real.

What practices are used to make different characters speak differently?

I can't bear the thought of reverting to crutches such as speech impediments, favored swear-words, or noticeable accents.

And yet, I've read far too many books where I felt that the characters all spoke alike.

How can I make the differences be subtle, so that:

1. Characters don't sound the same; and yet-


2. The difference sounds natural, and not as if I'm forcing the characters to talk in a way they shouldn't?

Note: My question is referring to speech patterns only, not to character behavior.

Answer

Consider the different characters':

• Level of intelligence
  
  
  
  • stupid characters contribute stupid thoughts to conversation
  
  
  • smart characters might only contribute when they know they have something important to say
  
  
  
  


• Interest in the conversation



• Social personality: whether introverted or extraverted


• Subject matter of interest (since they would continually bring it up):
  
  
  
  • A priest constantly going back to the bible
  
  
  • A war veteran who relates everything to "the war"
  
  
  • A player constantly talking about women
  
  
  
  


• Level of engagement with the conversation's subject matter


• Use of gestures with speaking



• Relationship to the other characters speaking:
  
  
  
  • Their reactions, both physical and in dialogue to the other speakers
  
  
  • Any subtext brought about by the relationship(s)
  
  
  
  

Give each character about 4-5 "characteristic" phrases they use often. Eg:

• "Seems like"



• "Bloody"


• "Just so"


• "Theoretically"

Vary the sentence length between characters