Natalie Wood died before filming was complete, thus the ending had to be constructed from scenes shot earlier. The film was dedicated to her memory.
When actress Natalie Wood died near the end of principal photography, studio executives tried to kill the film and claim the insurance, saying that director Douglas Trumbull could not complete the film. However, Trumbull's contract gave that decision to him, and he insisted on completing the film, using a stand-in and changing camera angles for the few remaining shots of Wood's character. The resulting hostility between Trumbull and the studio executives meant that this would be Trumbull's last Hollywood film. He has since devoted his efforts to effects work for IMAX films, theme park rides and the like.
The shots of "Heaven" were done by poking a hole in a piece of sheet steel, putting a quartz bulb behind it, and a piece of very scratched plastic in front of it. The rivers of "angels" were made with high-speed, backlit footage of a dancer wearing a long costume which was twirled about on two poles.
The tape used in the tape machines is a variety of decorative tape made by 3M. 3M only sold it in four-inch widths, so it had to be slit by hand to two-inch widths to fit in the tape machines. When filmed, they were astounded at how gaudy it looked, so to dampen its brightness, the prop crew wound the tape back and forth across a sander to dull its brilliance. "One of those things that actually looked a lot better on film when we finished with it," Douglas Trumbull commented.
Douglas Trumbull originally wanted to film this movie in "Showscan", a 60-frame-per-second widescreen process he'd developed, but the costs of retrofitting theaters to show it proved prohibitive. If the "Showscan" version had been made, each non-"Brainstorm" frame would have been printed twice to create a 30-frame-per-second "normal" film rate to compliment the cropped, non-widescreen shots. The intent was to create an experience similar to what the onscreen characters were "viewing."
Several more shots of "Hell" were created but cut from the final print for pacing. One of these was termed "Condo Hell" and featured many rapidly changing faces drifting slowly towards the viewer against an austere, high-tech looking background. Some of this can be seen in the trailer for the film. ("Cancer Hell" was the crew term for the gory-looking shots of tormented souls)
Lillian's line to Alex "Your hotshots Evans and Wetmore failed on that one year after year" is a reference to the film's consulting engineer Evans Wetmore.
When Michael first shows the machine to his son Chris, he calls it "The Hat." However in the script (final revised draft 9-21-81) he called it a "super-conducting phased field neural discharge sensor array."