From NetFlix:

Sean Penn (in an Oscar-winning role) stars in this fact-based drama about
Harvey Milk, the openly gay activist and San Francisco politician who was
murdered along with mayor George Moscone (Victor Garber) by disgruntled city
employee Dan White (Josh Brolin, in an Oscar-nominated role) in 1978. Director
Gus Van Sant’s compelling biopic (nominated for a Best Picture Academy Award)
co-stars Emile Hirsch, James Franco and Diego Luna.

Where to begin! That’s easy: First you could watch The Times of Harvey Milk (1984) which is an excellent documentary. More than that, ‘Milk’ often intersperses parts of the documentary into the film. Secondly, there is an excellent Wikipedia article on Harvey Milk which is long and very detailed.

I worry about a film which is an historical interpretation lest it become an historical distortion. For example, at one point Sean Penn says he thinks Dan White is really a closeted gay. Did the real Milk ever say such a thing? If not, I object to inserting such a gratuitous comment. However, the Wikipedia article together with the documentary lend much credibility to the Hollywood version. The article more than the film shows Harvey Milk as a complicated man who in fact did have many sexual affairs, who had a bit of an an attitude and could be very difficult to get along with, had several lovers involved with suicides (threatened and real), and who in his 40s was pushed into reacting and becoming involved politically by external forces. Granted any film has to focus on some aspect of the subject and there is too much to tell about Milk and his colorful era to put into one film.

Sean Penn captured Harvey Milk surprisingly close in looks but especially in manner as the documentary shows the ever smiling Milk. Josh Brolin looked like and captured Dan White to a remarkable degree. In fact, the very end of the film pairs photos of actors with their real-life counterparts, and the similarities are often striking.

The documentary (and, in all fairness, the film) made it clear that the double murder was meticulously planned. I wish the film had explained more what a joke the murder trial was (e.g. gays were excluded from the jury). The Wikipedia article explains how the “twinkie defense” motivated subsequent California laws which abolished “diminished capacity” as a defense to a charge.