Ever notice how addictive some characters speech patterns become? One of my characters has a distinctive way of talking. It's central to the overall backstory for them. I have to catch myself and avoid using the same style for others, sometimes!
Yes, know what you mean. I was saying to a mate of mine yesterday that something I have found really made a difference to me was the transcribing work I do for her - listening to people talking about themselves at length really shows up all the different little verbal tics and punctuations people use. Some people speak slowly and measure every word, others seem to have to rev up for every sentence with a load of 'yah, and, well, erm, so, the thing, the thing is...'
I suppose it might be a helpful exercise to do something like listen to discussions on the radio and try transcribing them, just to get a feel for speech rhythms. <has idea to pass on to writing magazine, no worse than a lot of the advice they give to the aspiring...>
I like to imagine my characters are being interviewed for a radio show, and I ask them each the same questions. Their answers highlight their different speech patterns as well as develop their point of view.