Herrmann has many such moments in the film, but the most important, in terms of the larger themes and complicated plot Beatty needed to make clear, comes in the scene when Max Eastman goes to greet John Reed on Reed's return to America after being in Russia reporting on the Revolution.
In that line reading we see multiple things: we see Max Eastman's entire experience while John Reed was away (and, by extension, the experience of the rest of the community). We also see that a sad resignation has come over Max Eastman, an acceptance of the harsh reality of the present moment. John Reed is still capable of being outraged, blustering at his friend, "Can they do that?" Eastman has been chastened, crushed a little bit. Eastman knows Reed will have to go through the rude awakening that he himself has gone through. So he has sympathy for Reed in the line reading too, the sympathy of the man who has understood a terrible truth and waits, patiently, for his friend to get it.
Edward Hermann was a character actor who constantly worked, not at all a surprise considering his brief comments above about Beatty's direction and how he understood it, and how it helped him know how to play his role. There was an intelligence and a total lack of ego in Herrmann's approach.
While many were referencing his recurring role in "Gilmore Girls" when they heard of Herrmann's death, the first thing I thought of when I heard of his death was the sad and knowing expression on his face in "Reds" as he watched his friend thrash angrily towards him. The expression on his face came from an acceptance of rancorous ugliness, and mourning that his friend's idealism is about to be shattered, just as his has been. Edward Hermann was able to deliver a simple line like "Welcome home" and somehow put the entire history of the Socialist Left in America into it. That's a character actor.