During the jail cell scene, Joaquin Phoenix breaks a real toilet. His actions were entirely improvised. Due to the historical past of the building where the scene took place, the toilet was considered "historical." Joaquin had no intentions to break the toilet, nor did he think it was possible.
After a few days of shooting, Paul Thomas Anderson noticed that Joaquin Phoenix would use the whole space where they were filming, even if it was outside the marks, so he told the lightning crew to have the whole space ready to follow Phoenix with the camera if necessary.
Paul Thomas Anderson has acknowledged that L. Ron Hubbard and the Church of Scientology served as partial basis for the character of Lancaster Dodd and his cult, "The Cause." This revelation sparked much discussion in the press, as the Church of Scientology has a long history of litigation against critics of Hubbard. Though the Church released no official comment on the film, Anderson claims that when he screened the film for his friend Tom Cruise an outspoken Scientology advocate, Cruse erupted with anger, specifically objecting to scenes where Dodd's son Val admits that Dodd made up the tenants of "The Cause," which parallel's real-life admissions by Hubbard's son. Anderson has admitted to a heated exchange with Cruise, though both the actor and director have kept details of their argument, and the outcome private.
The trailers for the film were edited by Paul Thomas Anderson himself without any permission from the studio. The trailers were notable for consisting mostly of footage not featured in the final cut of the film.
The primary influence for the film was John Huston's 1946 Documentary: Let There Be Light (1946). Director Paul Thomas Anderson has stated that he was taking material from the film "left, right, and center". Producer JoAnne Sellar has also stated that they also used the film as a reference for production design and costume design.
Paul Thomas Anderson initially wanted to put his own children in the film during the early scenes set at the portrait studio, but he decided against it because his mixed-race children did not have a period appropriate look.
Jeremy Renner was originally going to play Freddy when the film was prepped to shoot years ago, but when financing fell through, and Joaquin Phoenix was available following his I'm Still Here (2010) project, the filmmakers were able to cast him.
The first motion picture in 16 years to be filmed entirely on 65mm format using Panavision's System 65 camera. This process can be projected into 70mm. The last full-length motion picture to be shot in 65/70mm was Kenneth Branagh's 1996 adaptation of William Shakespeare's Hamlet (1996).
Paul Thomas Anderson wanted a workprint to match the Avid cut, so he had his editors assemble the physical film daily to match the digital edit by hand. Anderson also bypassed the use of a Digital Intermediate instead color grading the film with a photochemical timer, which resulted in a higher film resolution due to less manipulation of the filmed image.
The decision to shoot in 65mm came from a desire to replicate the look of photos taken by vintage Pressman cameras, which use large-format 4x5-inch film. This also led to the use of the narrower 1.85:1 aspect ratio (65mm has a native aspect ratio of 2.2:1). Anderson initially suggested shooting the film in VistaVision, and test footage was shot in that format, but the shallow-focus effect was not pronounced enough.