Leo Buchanan

Leo Buchanan appears on the edges of the world Mindhunter builds around its investigators, a young presence caught up in the ordinary unease of a household under strain. He watches more than he speaks, absorbing tension he doesn't fully understand, wary of the adults around him and quietly resistant to being managed.

O'Neill plays him with an unnerving stillness, the kind of childhood watchfulness the show trades in elsewhere with grown men who study violence for a living. He never overplays a scene; the discomfort comes from what he withholds, letting silence do the work a script full of dialogue couldn't.