“They only need to know one of us,” Harvey Milk explains to his campaign team in the film “Milk.” Sean Penn’s performance as Harvey does exactly that: You learn one man’s story, and his pains and triumphs become your own. It showed me how a great performance can also be a humanitarian act. When we know one character, one story, we recognize him as being of our own flesh and blood. When we understand his feelings, we put ourselves in his position. Not only is Sean’s performance honestly and lovingly humane, but it is also virtuosic — every note is so subtly tuned that the work behind it is never visible. He infuses Harvey’s courage with cowardice and his sexual prowess with hesitation. Sean’s Harvey is a cocky and charismatic orator, but always weighted by the foreboding dread of knowing his own tragedy. When the antigay Prop. 6 is unexpectedly voted down, surprise, elation and horror at the very existence of the referendum all rage in the blood beneath his skin. Sean Penn so inhabits Harvey Milk that I left the theater feeling the need to march against our frighteningly similar Prop. 8 to honor this man I now know.