- Acting for Two: Paul and Eli Sunday are both portrayed by Paul Dano, which causes some confusion.
- The Danza: The main character is played by Daniel "Plainview" Day-Lewis, and Paul Dano plays both Paul Sunday and his twin brother Eli.
- Disabled Character, Disabled Actor: Deaf actor Russell Harvard as an older H.W. Plainview.
- Dueling Movies: With No Country for Old Men, which was being shot around the same time in rural Texas, note was also a Darker and Edgier film by (a) respected auteur(s), released around the same time by the same distributors (Paramount Vantage & Miramax) and competed for Best Picture at the Oscars.
- Enforced Method Acting:
- Daniel Day-Lewis, naturally, went to town. He hit Paul Dano earlier than the intended mark during the scene where Daniel screams at Eli to heal his son. The look of shock when Eli tumbles to the ground is genuine. His hatred of Eli, which he continued even off-camera, led Paul Dano to confess he thought he would actually die during some points of the filming.
- During the baptism scene, Paul Dano is actually smacking Daniel Day-Lewis around, bringing it around full-circle.
- Fake American: Kensington-born Daniel Day-Lewis plays the American Daniel Plainview and Belfast-born Ciarán Hinds portrays Fletcher Hamilton.
- The Other Marty: Kel O'Neill, the original actor for Eli left halfway through production.
- Playing Their Own Twin: Paul Dano, originally cast as Paul Sunday, took the larger role of Eli Sunday (making the brothers twins) after Kel O'Neill departed the production.
Trivia / There Will Be Blood