A common culprit that keeps writers from making their work quality material is characters that sound exactly alike. Remember, each character in your manuscript is a living, breathing, thinking person with different wants, needs, and point of view from the others. A good exercise to fleshing out characters is to figure out what each character's super objective, figure out what a character truly wants in life (not necessarily in the story). These are the big things, the ones in our very core – to love, to be loved, to be powerful, to be respected, etc. Once you figure that out, realize that this is JUST to determine their core character – how they approach every situation and character they encounter during the course of your story. It's the foundation, and while it's certainly the most important layer, there are more layers: the style, and the details.
A character's style is not about their fashion, but about how, knowing their core, they approach life and other people. Things like humor, vanity, selfishness, selflessness, etc. You can think of a character's style as a collection of their coping and defense mechanisms. How they get by on their day to day life.
The details are how, knowing their core and their style, what the little actions are that they take frequently. For instance, if he drinks a lot, or is always fixing his hair or keeps a pack of cigarettes rolled up in his sleeve – even though he never actually smokes. Each person has their own unique tics, between the character's roundabout way of parsing out information, their distinct voices from each other (stemming from different wants), and the dialogue feeding into the theme – each of those individually are subtext, but the fact that all three are present clues the reader in that the writer is a professional.