In the Blatty novel and in the Blatty-Friedkin film, Merrin's 12-year prior exorcism "lasted for months" and "damn near killed him".
The first sequel, Exorcist II: the Heretic, preposterously shows Merrin hauling a possessed boy up a cliff, which is followed by a very tame, perfunctory exorcism with no paranormalia evident. If Merrin's heart was already weak, surely the climb-cum-exorcism would have been dangerous for him. But Boorman gives us no indication that Merrin's heart is weak during this first exorcism, and Merrin himself seems perfectly fit at this moment. He only "gains" his bad heart later on, when the faux-Blair double metaphorically crushes the priest's diseased organ (his heart, be it re-affirmed for the dirty-minded).
In Exorcist: the Beginning, the same principle applies. Merrin is forced to crawl through tunnels and is tossed around telekinetically, but is amazingly resilient. Even the loss of his love interest seems a momentary, non-life-changing grief.
In Dominion, Prequel to the Exorcist, we get more of the same. Merrin confronts the demon in the underground "chapel", but again he is amazingly resilient and shows no sign of the cardiac condition that will eventually kill him. Here, too, Merrin gets bounced around a bit, and he is given the grim and certain knowledge that even had he acted against the Nazis, the outcome would have been the same. He merely endures one or two physical shocks and a psychic trauma. The exorcism itself only lasts minutes, and aside from a faceful of vomited-forth stinging quasi-insects, Merrin remains quite intact, physically and mentally. Moreover, he emerges much stronger spiritually.
None of these films does justice to the original exorcism as described by Blatty. Had the filmmakers and screenwriters simply run out of terrifying ideas? Surely they could have obtained some very creepy case information from Blatty's novel, which describes the horrendous things that are purported to have happened during exorcisms. But they treat the exorcisms like something they want to present as briefly and as shallowly as possible.
Blatty's film Exorcist III: Legion is the only "franchise" film – other than, of course, the original film – that suggests the exhausting nature of exorcism. Its exorcist, Father Paul Morning, is subjected to extreme physical and mental torture. Although over-the-top, Legion's presentation of demonic assault is harrowing. One can easily imagine physical and mental damage resulting from a Legion-type exorcism, especially if it extended over months.
So .. with all the material available to the subsequent films, it is surprising and disappointing that they never match the fury of the original story's/film's exorcistic depictions.
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) … the follow-up films had no cognitive meaning other than being cash-ins on the box-office success of the one and only film, The Exorcist. Had Morgan Creek stayed out of Blatty's way, his true second film, Legion, could have had some potential.