Why this is a mistake: Because you are writing all your characters,
it’s easy to make them all seem very similar to one another. They tend to talk alike, and their points of view seem to be the same. This is because most people have a hard time getting out of themselves
and into their characters. Your reader must feel that the characters
in your story are distinct.
The solution: Use real people (with details changed), archetypes, templates, profiling, or any other approach that breaks people down into different types. Once you have created distinct characters, make sure you don’t start melding them together inside of the plot.
To ensure every character’s dialogue is distinctive, highlight each character’s dialogue with a different color. Then read each color one at a time to check for consistency.