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 movie was to find people "who were not working out all the time, and who still had pubic hair."
No authentic Woodstock footage was inserted into the movie. According to director Ang Lee, all the crowd and concert footage was shot specifically for the movie.
In the movie's ending, 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.
The cost of a ticket to attend the Woodstock Music and Art Fair was $18 in advance, and $24 for all three days.
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.