Nobel Prize-winning physicist Ernest Lawrence was one of Oppenheimer's closest friends and, of course, a key figure in the Manhattan Project. Josh Harnett, plays the part with enough warmth and a lot of conviction. With a lot of screen time and his acting skills, Harnett makes a lot of impact as Lawrence. The character has a lot of dialogue, but his most memorable scene is the silent one where he walks away without giving testimony against his friend, Oppenheimer, at the very end of the movie.

Source: https://fugitives.com/oppenheimer-characters-ranked-in-terms-of-impact/