Dialogue needs to read like you actually speak, not like you think people speak. People are rarely articulate and fluent in everyday life. Don’t be afraid of jumbled sentences and pauses. Read every line out loud. If it sounds forced or cheesy, rough it up.
Following on from what Lisa said, as well as not speaking in full sentences, often people don’t fluidly respond to each other’s statements or questions. There are thoughts in between that aren’t articulated and so lead to tangents, or they completely ignore what the other people / person has said. Some people repeat what the other person has said—particularly if they are struggling to comprehend the information that has been relayed. Or if they’re trying to steal someone’s idea & claim it as their own, they can dismiss it then repeat it with a slight alteration. Also, do be careful of putting in too many colloquial speech patters / pronunciations or too many ums & ers in an attempt to be genuine. If you over do it, it can be off-putting to read.
Less is very often more. And the silences are just as important as the words. Make every word count, it has to earn its place.
One of the key things I always make myself do with any dialogue I write is read it out loud. This may convince people who you live with that you’re finally losing the plot (excuse the pun) but it’s a sure way to work out which bits of dialogue are completely inauthentic once they have an actual voice behind them, as opposed to being flat words on a page. It also helps you to get into your character’s head and really put yourself in their shoes. That aside, I think the best way to write good dialogue is to invest a lot of time just randomnly listening to people around you. Not just how people speak and what they say but also how body language and mannerisms weave into conversations. Sprinkling those things in as part of your dialogue can tell readers a whole lot more than just the words coming out of your character’s mouth.
People speak differently, whether it’s certain phrases or the way they structure their sentences. Not everyone has a catchphrase, so don’t just attach one to each character when they speak. Listen to the way people talk in real life and note the differences and similarities in their speech. Friends sometimes adopt the same word choices without even knowing it. One of the key things about dialogue is to write it in a way that feels authentic, while also incorporating the character’s voice so that the reader can tell who is speaking without having to be told.
People NEVER say what they mean. Don’t try to force a subtext, if you let the characters flow then the subtext will be there. For example nobody ever says “i love you” and means “i love you”. When people say “i love you” they mean “i’m sorry”, “i’m scared”, “don’t fuck this up” or all kinds of other things.
Pick a character and read only their lines out in your MS, it will help you see if their speech is consistent throughout. Don’t forget what age they are, gender, etc, as well as including their personality traits, this should all come across in their dialogue. People often skirt about what they’re trying to say, only hear what they want to hear and don’t actually say what they mean. Make it clear who is speaking especially in children’s writing. Stick with using ‘said’, don’t clutter with dialogue tags. So saying, for really effective dialogue you should be able to remove the named speaker and still be able to work out who is saying what. Easier said than done.
For me, dialogue starts with getting to know your characters. When you know your characters well enough to step inside their skin, the dialogue will flow. And if we think of our characters as real people, we usually know what they will say and how, whatever the conflict. Do the basics too, read your dialogue aloud and ask yourself, have you been honest and fair to your character? What happened to them last and what might be going on inside their heads? Will this influence them? Also think about rhythm if you can. Lots of heavy, dense dialogue will need the occasional light moment. Not easy but trust yourself – you’ll know when you’ve got it right.