According to The Washington Post, Screenwriter and Producer James Schamus told reporters at the Cannes Film Festival, that the biggest challenge in casting extras for the film, was to find people "who were not working out all the time, and who still had pubic hair."
In the end of the film, Michael Lang says he's going to San Francisco to organize a free concert with the Rolling Stones. He's referring to the infamous Altamont Free Concert, where four people ended up dead, one of them by homicide. Because of these violent events, the concert was later on viewed as the end of the hippie era.
Although no actual Woodstock footage was used, the movie included re-enacted scenes of the original Woodstock movie being made, as background to the action. Two examples are the film crew on the road to the festival getting the walking nuns to flash the peace sign, and the toilet-cleaning "Port-O-San" man being interviewed.
In the movie, the song "Maggie M'Gill" by The Doors, is played. However, The Doors turned down the opportunity to play at Woodstock. In fact, on the movies Official Soundtrack, the only group or artist featured that did not play at Woodstock is The Doors, aside from Danny Elfman.