Listen carefully to the way people really talk. You will find repetition, unfinished thoughts, and asides. I often read the transcripts from news talk shows. Those really show how a straightforward speech isn't straight by any means. Not that what the commentator is true, it just comes out as a bit bumpy. And these people are trained to talk! So when you write dialog, don't forget to add a bit of human error. Not too much, just a bit.
"Yes, yes, I'm sure they did. But that is what alerts us to the fact that they are not . . . rather, what they appear to be doing is merely a cover for what they are really doing."