I've been told that I have a good ear for dialogue. This is encouraging, given that I still harbor a secret wish that I might someday write a successful screenplay, and screenplays are largely based on dialogue. It is not, however, enough to carry me through the act of writing dialogue for fiction.
My problem is that I let characters talk too much, and usually about the wrong things. Instead of focusing on the issue at hand, I give them topics like work, play, whatever. I need to learn to rein myself in a bit, narrow my field of vision, home in on the necessary details.
Fiction is never going to be hyper-realistic, even if it appears thus. Because it's a world created from scratch, and so I can't, uh, talk like this or, you know, like a bumbling moron because, um, that would be bad, right?
Maybe I should borrow the famed expression "loose lips sink ships": talk too much, and the ship of your characters' conversation will succumb to the sea around it, sending it to the bottom of the ocean where it will be inaccessible. (I'm bad with metaphors, but you get the idea.) This seems like a reasonable piece of advice to me.