There are a couple of points I’d make about realistic dialogue.
Firstly in any medium the characters don’t talk like real people. If they are well written they talk like real people without all the boring bits, Listen to real conversations and you’ll see that people say far more than is necessary, in confusing, convoluted ways often straying from the point. A writer distills all this stuff so that his characters talk in ways that either reveal their character or advance the plot. Some do the distillation better than others. Just sit around and listen to others talking some time and imagine how interesting a transcript would or wouldn’t be.
I am one of the people who disliked Juno for the cutesy dialogue stylization, but not because it was unrealistic (I quite liked it in those terms) but because every character seemed to sound the same. I just ended up with a sense of the writer talking through her characters rather than the characters having voices of their own.