Everyone with any real personality or presence crafts the substance of their own reverberation. Y'know what I'm talking about? That feeling you get when a person walks into the room, their vibrations carried along by their gestures, the sound of their voice, the color of their eyes, etcetera. Conveying the feeling of a fictional character's echo without making the piece needlessly wordy is an interesting challenge. Too many details can mislead the reader, while too few can allow the character to operate under false pretenses and come across false to the audience.
Running down a subconscious or knee-jerk emotional response to every character that graces the page shouldn't be necessary, but I find myself thinking about these things as I write. What do they smell like, are their hands rough or smooth, do they button their shirt sleeves, does their handbag look gaunt and starved... or overflowing... with stuff. So I set about making my small sketches, and writing a first impression page for fictional people... all eccentricities intact. In my own life this attention to detail is a reminder that at heart I'm probably a detective or a lawyer playing at being a writer... I don't care.
In the real world the echo a person cultivates or crafts comes from a host of personality traits, conscious and subconscious. Pride, self-awareness, brutal introspection (my favorite), and so forth, all contributing to how a person interacts verbally and non-verbally with others and their environment. Now, the way these echoes commingle on the page when two strong characters meet for the first time, verbally spar, or fall in love can be dramatic, interesting, and provocative. I love using words to weave the ethereal elements of two or more characters... it makes this difficult process so worthwhile.