Ashley Bell did all the bends and contortions of her body on her own. No special effects were used because she has hypermobility (an exaggerated increase mobility or loose joints more flexible than ordinary), a very common problem which affect more than 15% of the world population, without it involves a pathology.
The scene in which Nell is given a pair of red Doc Marten boots stemmed from the fact that the insurance company refused to cover Ashley Bell for her exorcism scenes if she was running around barefoot.
Note how much of the footage at the start of the film is fairly static (tripods were used for the early scenes). As the film progresses - and the documentary crew start to become more fearful - the camerawork becomes much looser and unhinged.
Was in profits before shooting even began. Producers Eli Roth, StudioCanal, and Strike Entertainment sold off a few key territories to finance the film based on the script and the producers' names and Roth's commitments to do publicity, raising $2 million and making the film for $1.8 million.
A crew member's brother was a real exorcist, and was on set advising director Daniel Stamm. He makes an unbilled cameo in the film and can be seen to the left of Reverend Cotton Marcus when he delivers his sermon about the Prince of Lies.