During the filming, director John Boorman contracted San Joaquin Valley Fever (a respiratory fungal infection), which caused filming to be suspended for five weeks. It was determined to be caused from the dust used in the African sets from the film.
Jon Voight originally agreed to play Father Lamont before dropping out over concerns with the screenplay. David Carradine and Jack Nicholson were considered to play the part: the studio rejected Carradine due to an ongoing dispute with him over the Kung Fu TV series, while Nicholson's salary demands were deemed too high.
Originally, the script had a major role for Lee J. Cobb's character of "Lieutenant Kinderman" from the first film, but upon his death the entire film was reworked. The script for "The Heretic" apparently bore no resemblance to William Peter Blatty's "Legion", which was later made into "Exorcist III" and featured "Lieutenant Kinderman" (then played by George C. Scott) in a starring role.
When the role of Dr. Tuskin was still written as male, Chris Sarandon, George Segal, and David Carradine were considered. John Boorman passed on Sarandon, and George Segal's salary demands were too high. When the decision was made to make Dr. Tuskin a woman, the screenwriter suggested Jane Fonda or Ann-Margret before Boorman decided upon casting Louise Fletcher.
The swarms of locusts were realized by painting a few thousand Styrofoam packing peanuts brown and shooting them out of a large air blower. Director John Boorman had experimented with a number of techniques to get actual grasshoppers to swarm around (including clipping their legs off so they couldn't land!), but none were convincing enough for him, so they used the peanuts (nicknamed "Larrys" by the crew).
In one of the last scenes, when the house is shaking and the bed rolling, Linda Blair fell off of the bed into a crack in the floor. Viewing dailies showed this to be a superior way of dealing with Blair's character, but a stage hand jumped into the scene to rescue Blair from injury and the shot was unusable.
Although widely regarded as a flop, the film was in actuality the only one of the sequels/prequels to show a profit on its theatrical release despite being a major underperformer and a critical embarrassment to the studio. However, the films reception was so bad that plans for a second sequel were promptly dropped.
The original cast and crew of The Exorcist were originally very much opposed to a sequel. William Friedkin and William Peter Blatty actually met to discuss ideas at one point, but when they failed to develop a suitable premise, they abandoned the project. Both 'Linda Blair' and Ellen Burstyn turned down repeated offers by the studio, though Blair eventually agreed to return when presented with what she considered a good script. However, according to Blair, due to various rewrites the script ended up a total mess. By that point, however, she was contractually bound to a sequel, and unable to drop out of the project.
John Boorman's autobiography recalls an anecdote in which he claims Linda Blair came up to him one day during the beleaguered shoot and perkily announced, "without irony," "Did they tell you? I was only ten minutes late this morning!"
The trivia item below may give away important plot points.
Despite Ellen Burstyn's repeated refusals of the offer to return, most drafts of the screenplay featured Chris in a central role, eventually dying at the end of the film and passing Regan's custody onto Father Lamont. It wasn't until shortly before shooting and Kitty Winn agreeing to return that her role was swapped for that of Sharon.