Dialogue seems to be one of the hardest parts of writing to get right. It doesn't seem like it would be too hard. I mean, everyone listens to people talking all the time. But I guess looking at people's faces every day doesn't make you a portrait artist, either.

It seems to me that you have to strike a delicate balance between what people actually sound like when they're talking and what your characters can say that's interesting or funny enough not to bore your audience to tears.

Chekov, for example, is too heavy on the former. Lots of long pauses and awkward moments which, yes, mimic real human conversations faithfully, but also put people to sleep. On the other end of the spectrum are most modern movie scripts, where everyone speaks in catch-phrases and stupid puns. Most fiction falls somewhere in between.

A good author can pen a conversation between characters that not only conveys plot development and needed information, but does it in an interesting way, while also managing to sound like natural speech.