Thank You, Plato
I'm about 95% sure that if you're reading this, that you know who Plato was. You may not know much about him, but surely you know he was a philosopher. For a short history lesson, he was one of the original philosophers from ancient Greek. Born in Athens in 427 BC, he spent much of life under the tutelage of Socrates. Like Socrates, he spent that time thinking, philosophizing, pondering. (While that may sound like a good job, understand that Socrates went to trial for corrupting the youths of Athens. He was found guilty and put to death.)
What does Plato have to do with writing?! Well, I'm getting to that! As you've probably found, one of the more difficult things when writing a book , is to keep the distinctness and feel of the characters separate from each other. I recall early on while attempting to write my first books, all the characters felt like me. But I was able to overcome that with the help of Plato.
Plato's Forms
Plato had this philosophical theory that the physical world is not as real or as timeless as it appears. Things that appear before us are just imitations of the real thing. Think of a circle. It cannot exist in the physical world because it is a mathematical entity. As soon as it is created in the physical world, it's imperfect. There is the idea of a circle and the form of a circle. The idea would be the mathematical concept. The representation in the physical world would be the form of a circle. Plato's idea was that a circle is not a circle until the idea of a circle is in the form of a circle. That's probably a little hard to conceive, so I'll use the example my philosophy professor used: a dog. Like the idea of a circle, my professor referred to the idea part of a dog as the dogness. Consider that everything there is about a dog: its personality, its way for sniffing everything, how it wags its tail when it sees you when you get home at night, and so on. All of that would be referred to its dogness. Next, consider the actual physical representation of the dog: its hair, it's bronze hair (if it's a Golden Retriever), its narrow muzzle, its floppy ears. All of that would be referred to as the form of the dog. What Plato's Forms state then is this: a dog is not a dog until the dogness of the dog is in the form of the dog. So you take all that personality and wagging of its tail and put it into the physical form of the dog, and you finally get the dog.
What Am I Driving At
So what I"m driving at is how you approach characters in your book. Over time, by learning to take on the idea and form of a person you are writing, you will be able to change from that character to another with ease. It does take time, but the more you do this, the more you are able to have whole conversations between two opposing characters and make it sound not only plausible but good.