I do have some thoughts on dialogue and what makes it sound realistic, particularly in historical fiction. The first two notes sound easy, though they take practice. The third one takes work.
1. Listen hard.
I’m one of those (likely annoying) writers who often works on the sense that the characters, and the story, exist somewhere in the ether, and it’s my job to transcribe as best I can. I find that if my dialogue keeps coming out stilted, I’m trying to force them to do something that’s not consistent with their character or situation (or not what they want to do). I get into their heads and listen harder, and it often smoothes out.
2. Read it aloud.
Even act it out a little. Read it as though you were reading it at a storytelling event, or for kids (if it’s not a graphic scene, but of course). You may _hear_ what’s missing or what’s too much when it’s spoken.
3. The most important “rule” I try to keep in mind: BALANCE.
A lot of time when dialogue feels stilted and awkward to me, when I’m writing OR reading, it’s because that’s all that’s going on. The writer has something particular that needs to be expressed (or the characters do), and that’s all we see. That’s where we get into he said/she said, simple exchanges back-and-forth.
But real-life dialogue isn’t like that, is it? Your husband says something, and while he’s talking you notice that the wind’s picked up outside, and you think you need to move the chairs before they blow over. You may say that next, before you respond to his point. Or you say something to your child, how naughty she’s being, but at the same time you’re gently wiping the food off her face. There are thoughts, actions, and changes in focus that go on—and to me, balancing these things in a fictive dialogue makes it FAR more interesting.
Show a difference between the words spoken and the thoughts—this gives depth, and shows subtexts. Change the focus for a minute (the wind example above)—this gives the reader perspective, and sometimes a breather, particularly if it’s an intense conversation. Put in actions that are direct reflections of the conversation, AND as well, some that are not, but are reflective of other things going on. Also, don’t forget to add different senses into the mix. All of this makes a dialogue far more “real”.
4. Another note, about individualizing characters in historical fiction specifically. Be wary with accents and historical words; they’re easy to overdo, and then your dialogue just is hard to make out, or sounds ridiculous. Sprinkle things in. But more interestingly, use metaphors particular to your character and/or time period. (My MC’s father, a mason, says: “Clearly I should not have trusted a girl of sixteen summers to have sense enough to lay one stone over the other.” This makes sense for him, but isn’t something any of the other characters would say.) Also, use different rhythms and sentence structure to distinguish different people, rather than just accents.
Medieval Word of the Day: runkle (Scots and north): a wrinkle or crease.