Dillon Freasier (who plays H.W. Plainview, the son of the character played by Sir Daniel Day-Lewis) was not an actor; he was an elementary student near the film's West Texas shooting location. On the radio program "Fresh Air with Terry Gross," Paul Thomas Anderson told Gross that when the production was trying to convince Dillon's mother to allow Dillon to be in the movie, his mother wanted to figure out who Day-Lewis was, so she rented a copy of Gangs of New York (2002) (in which Day-Lewis plays a murderous gang leader nicknamed "The Butcher"). She panicked at the idea of her son spending time with the man she saw in that movie, so the 'There Will Be Blood' casting department rushed to her a copy of Le Temps de l'innocence (1993), in which Day-Lewis plays a civilized and gentle man.
While on location in Marfa, Texas, No Country for Old Men (2007) was the neighboring film production. One day, Paul Thomas Anderson and his crew tested the pyrotechnical effects of the oil derrick fire, causing an enormous billowing of smoke, intruding on the shot that Joel Coen and Ethan Coen were shooting. This caused them to delay filming until the next day when the smoke dissipated. Both this film and No Country for Old Men (2007) would eventually become the leading contenders at the Academy Awards a year and a half later.
In an interview on the National Public Radio program "Fresh Air with Terry Gross," Paul Dano told Gross that he had originally been cast in the much smaller role of Paul Sunday, Eli's brother, and another actor had been cast as Eli. However, after Dano had already started filming his one scene as Paul Sunday, Paul Thomas Anderson decided to replace the actor playing Eli. Anderson then asked Dano to play Eli Sunday (a much bigger role) as well as Paul Sunday, and they decided to change the film to make the brothers identical twins. Anderson asked Dano to play Eli on a Thursday, and filming for the role began four days later, on the next Monday. Sir Daniel Day-Lewis, by contrast, had a whole year to prepare to play Daniel Plainview.
Paul Thomas Anderson told Entertainment Weekly magazine that the fake oil used throughout the movie included "the stuff they put in chocolate milkshakes at McDonald's."
Daniel Plainview bears some resemblance to a real, early twentieth-century California oil tycoon named Edward L. Doheny. Both were from Fond du Lac, Wisconsin; both were employed by Geological Survey and worked in Kansas; both tried a hand at mining before going into the oil business; and both worked with a fellow prospector named "H. B. Ailman." As for other Plainview-Doheny connections, the bowling alley scene in 'There Will Be Blood' was filmed at Greystone Manor, a California estate Doheny built as a present for his only son. Also interestingly, the infamous "milk-shake speech" Plainview gives is based on transcripts of congressive hearings concerning the Teapot Dome Scandal, in which the very same Edward L. Doheny had been accused of bribing a political official.