The mark of a good character is how they sound and hopefully different from other characters in the story. In the real world people have distinct speech patterns, favorite words and mannerisms when they talk that are different. The mark of a good story is when the reader can hear the character’s voice.
I grew up in the Midwest where we drank pop. When I lived in Florida people stared at me if I used the word, but mention soda and they knew exactly what I wanted. When I worked in North Carolina, I still remember when an associate asked if I wanted to get some sodie water. Words can vary from region to region and character to character.
Does the character speak in long sentences using big words, or simple phrases with one syllable words? The education level, age and who the character associates with affects how they talk. She talks one way at work but differently when having a drink with a friend. Lovers have their own language. A mother speaks differently with a child than a girlfriend or her husband.
Characters from different parts of the country have a unique sound. He may have a cracker accent or an easy southern drawl. A person from New Jersey sounds different than one from Iowa. Does his voice sound nasally, or low and slow?
Can you read the words and know immediately which characters are speaking? Is the dialog so clear that no way could they be mistaken for someone else? Do you hear their voices?