Right off the bat, it’s important to give your character, well, character.  Give them little quirks and habits to make them more realistic.  You know, like pushing their hair behind their ear when they’re nervous, or pushing their glasses up the bridge of their nose, rubbing their hands together, tapping their fingers on the table, etc.  Try not to just arbitrarily assign habits to a character; try to have them make sense with the character’s personality and back-story.  For instance, rubbing their hands together suggests a nervous or anxious-to-please personality, and if your character is more confident, that particular quirk might not fit well.  Or, if they used to be a musical performer, maybe they tap their feet and hum while they work.  You get the idea.