The bulk of the stuff I'm editing right now has a lot of dialogue, and that's been interesting to me.
I want the different characters to have different voices, but there are times I think I may have been trying too hard, so I changed them up. This is the good part of editing, especially with dialogue. When I read these conversations between characters out loud, sometimes I find myself saying "Well that guy sounds stupid."
Dialogue is a bit of a toughie. You really have to think it through. It has to be well-planned, but it can't sound staged or set up in anyway or it comes across as phony.
Also, now that I'm through the whole draft, I have a better handle on my characters than I did when I started, or even after I was going for awhile. That familiarity means I know who they are, and know what they would say. That helps with the editing of their dialogue in earlier chapters.
The words they say have to keep them within character, but they have to help develop them, too. This is a pretty tough balance - and it's a balance that isn't going to be achieved in just one edit.