As a few people didn't understand what I was talking about yesterdya.
One of the problems with protagonists is that you can be so focused towards moving towards the goal/exploring the conflict etc that the protagonist seems flat, like an actor rather than a person. The personality is missing.
Equally you might know lots about a character but unless it is down on the page, your reader won't know. Been there, done that. In my head, but not on the page = flat character for reader.
More than stories, readers remember characters. They keep mental files about what they look like or how they react to others. And you want to breathe life into your characters. They need personality, in particular your protagonist.
You also don't want your characters all sounding the same.
Some of the trouble is that the writer only has a limited experience. It is why listening to how people speak, the vocab that is used etc is so important. Little details that give insight. Telling emotional details that add a richness to the world.
The metaphors and how people describe each other are affected by their life experience. Even the pet phrases and the cliches. They help define personality and character.
In revision it is all about taking those bland phrases such as She looked at him and changing it to give a deepness and texture to the character.
She glared at his unshaven chin, grimy t-shirt and overly-pleased grin. How dare he come to her dinner party dressed like some refuge from a used clothes shop and expect her to hug him!
She raised her eyes to his intense emerald gaze. Nothing else mattered not the spreading puddle of water on her best Oriental carpet, the whispers of the servants, or even the impropriety of the hour.
The words and word choice help to give a roundness to the character. They revealed something about the POV character as well as the other. Same situation for both. Different responses. By adding the extra detail, you enrich the reader's experience and you make your character more rounded.
It is about attitude and opinion. Your protagonist has to care passionately about things and this helps to define your character and raise them from blandness. It helps to create characters who linger in your mind or who you feel have a life beyond the page.
Think about the characters you remember. Do you remember them because they are bland or inoffensive? Or do you remember them because they cared deeply and passionately? Or noticed things or somehow leaped from the page.
It is about making your characters memorable. And for me, with my protagonists, often it doesn't happen in the first draft. It may take several goes before I can those little telling details down that really differentiate them. Other authors can the details much more quickly. It is just how I work.
Another problem comes when you have delved to deeply with the same theme or similair characters. Where they were new and fresh once, you start falling back and reaching for familiar words/patterns. Results -- flat characters who all seem to be the same and do not have a life of their own.
And I can always strive to be better. Nobody said writing was easy!
Does this help?