What surprises about The Devil Inside is how its modern cinema verite style, interspersed with reality-show style after-action interviews, is combined with an increasingly unreal, illogical, and improbable story populated by very silly people and incompetent institutions to achieve (possibly unintended) comic effect.
Just take the film's basic set-up. Our very Gothic heroine – an insecure, obviously depressed, possibly psychologically disturbed young lady – manages to get the Vatican to approve her visit to her mum. She's never known the truth about mother's sanatorium treatment until a year ago – but the Vatican does! Which is (of course!) why the Vatican allows this young woman to lug a documentary maker with his camera into a maximum security mental ward, then buy a really bad excuse from her to turn off all the cameras in the ward so she can conduct a secret exorcism of her own! The cherry on top? The unsanctioned exorcism is conducted by two increasingly inept renegade priests who perform exorcisms without the approval and knowledge of the church, using no known proper exorcism ritual.
Of course the whole thing is botched in the end. But because the filmmakers don't really know the convention of demonic horror films (the most realistic and exhaustively researched, with actual knowledge of Catholic exorcism rituals being The Exorcist), the result is not at all scary.
However the script is so full of convenient coincidences and characters so incompetent that The Devil Inside might be greatly improved if its filmmakers had set out consciously to make the This is Spinal Tap of exorcism films. The Devil Inside is highly enjoyable because of and not in spite of its many flaws. Now if only the filmmakers had designed it to be a mockumentary instead. Ah, the missed chances!