Note the first:
When you change speakers, you start a new paragraph. Seriously, they should have taught you this one in grade school, or high school at least. I’m starting to suspect that it gets neglected because nobody expects most students to ever need to write dialogue. O tempora, O mores, what is the world coming to, and all that jazz.
Note the second:
When you’re writing a scene with a lot of dialogue, and feel the need to throw in small bits of action and stage business to break up the steady back-and-forth, or to show one speaker’s reaction to something the other person has said, the action bit goes with the dialogue belonging to the speaker who’s doing it. To illustrate:
Not like this:
“I don’t know what you mean,” Joe said. Jane looked at him with disbelief.
“Sure, you do.”
But like this:
“I don’t know what you mean,” Joe said.
Jane looked at him with disbelief. “Sure, you do.”
Don’t make your readers have to go through a scene’s dialogue twice in order to be sure of who is doing and saying what. Accidentally confusing your readers is bad.
Confusing your readers on purpose is a different kettle of fish. I personally don’t know why anyone would want to do it, but some writers do, and those writers have audiences, so if that’s your style, then go for it. But if you’re going down that path, not confusing anyone by accident becomes more important, rather than less.
Dr. Doyle's Blog 
Also annoying is when the dialogue goes on one line and the action goes on the next. It becomes hard to keep track of who is talking, but for a different reason.