"What is the use of a book," thought Alice, "without pictures and conversation?"
There aren't many books with pictures unless you include children's books and coffee table books; but almost every work of fiction has to have conversation which means the writer has got to get his/her head around writing realistic dialogue.
I've had an odd journey in this discipline. Way back, when the world was young, I wrote a great many plays (all awful, I'm sure). I wrote them specifically so my class could do a play every year. I wrote so I could act. Later, I wrote several duologues for drama festivals; again so I could act. I never paid that much attention to the fact I was writing; it was just a necessary step so I could get my feet onto a stage.
When I started writing my mystery games, my first commission was for a play. I was paralyzed with fright. By this time, I had acted for years and had said some of the best dialogue ever written. Who was I to think that I could write dialogue? I muddled through, gritting my teeth and was pleasantly surprised by the finished product. My customer told me the play was a big success. I still wasn't comfortable writing dialogue. I hadn't learned to trust my acting instincts would start to scream if a line was wrong. Over time (I've written one more play and many games with 'scripted' clues) I've learned to trust.
Writing dialogue is now one of the easiest things for me to do; in fact my WIP is very dialogue-driven. I know each of my characters so well that putting words into their mouths isn't a problem. I know their rhythms and their vocabulary. I've learned to write conversations without the annoying dialogue tags but still be able to differentiate between the two conversationalists. If a third or fourth person enters the conversation then the tags start to appear.
Any writer writes with their own voice, but each character must have their own voice too. I admire writers who can create characters who use identifying phrases (Hercule Poirot and his little grey cells) and who make their characters come alive through their words rather than by description.
It may be my theatrical and debating history, but I've always been interested in what people say and how each of us choose to phrase it. Get ten people together on a sunny day and all ten will describe the day differently. Vocabulary choices reveal so much; education, history, personal prejudices to name a few.
Every character in any book is there for a specific purpose. Let each of them speak with their own voice. Your work will be richer and more realistic. Do they stutter? Do they have the unfortunate habit of dropping a malaprop? Remember as you write; these characters aren't you, they need their own voice.