In my writing I am always searching for ways to make my characters more individual. After all, the only person I have to draw from is me so adding uniqueness to my characters can be a daunting task.
One thing that I have been trying to focus on is situational mannerisms. Physical reactions that we have to specific situations. I tend to suck my breath in loudly when I am surprised – a trait that my family finds entertaining. My husband does not suck in his breath when surprised.
While there are common reactions to surprise, fear, excitement, etc . . . it is unlikely that you would have two people in your story that have identical reactions to everything. (Unless their similarity is the unique aspect.)
Nervousness - Some bite their nails, pace, sweat, or prattle incessantly.
Anger - Some turn red, others white; shout, go silent; stare, can't make eye contact.
When you know what your character's reaction is, you can tell your readers how the character feels through the reaction and give them a familiarity with the individual at the same time.
Samuel dug the toe of his shoe into the dirt, looking down as heat flushed across his face.