The most important thing about dialogue is that it sounds natural. You need to have the feeling that it really comes out of that specific character. For that you need to know him or her very well.
And by natural I don’t mean the copy of a possible one-to-one conversation between two real people. If you listen to them you will find out how often people use filling words and say things three times, as if the person in front of them fell in a keg of tranquilizer.
You can’t do that in written dialogue. You can do it once to emphasize something; otherwise, you loose the reader’s attention. So listen very carefully how people in your surroundings talk. You will discover different rhythms and peculiarities, favorite words and filling words—even for yourself.
For research, it helps to go to a location or a community or a certain job environment and listen to the people there. I once overnighted in the Cologne subway. On a personal level so not recommended! But due the details I nailed it and the atmosphere of the short story was dead-on.
That’s why I will never write a legal or political thriller, because I find the language artificial and boring. I can’t imagine myself diving into that world and writing a whole novel about it. Although it is clearly bestseller material. So go for it if you can!
What drove me to stay overnight in subway zombieland instead? I sometimes wonder, too. But since I am neither a veteran nor a teacher ...
Here a real life example for subtext:
What is subtext? Someone says something but also means something else at the same time. Here is an example you probably won’t know:
Somebody tells you, “I would be glad to see your room all tidied up in the near future.” The obvious message is that this person would be happy if you, or maybe someone else, would tidy up your room sooner or later. But if not, that person would not burst into tears or have a tantrum. The reverse of the sentence, therefore, is that this person will still be satisfied, or even happy, if this doesn’t happen at all. Now let’s have a look at the subtext of that statement.
First of all, your room is expected to be tidied up. Nothing less. That means the person probably won’t raise her arms in jubilation or do a little happy dance because of the nicely folded clothes in the drawer. It is expected. Rather, she will shortly nod her head because only the expected happened.
Second of all, it is expected that you yourself, and nobody else, tidy up your room. Paying your 5 year old brother is not an option.
Third of all—very important—, the person won’t be happy, or even satisfied, if nothing has happened in your room since she was last there. She will likely go berserk.
That is subtext.