The film was shot entirely in Romania.

Taissa Farmiga, who played Sister Irene in this film, is the younger sister of Vera Farmiga in real life, who played Lorraine Warren in The Conjuring (2013) and The Conjuring 2 (2016). The Demon Nun, played by Bonnie Aarons, also appeared in The Conjuring 2 (2016). Though the sister's characters are not at all related to each other, they are both devout Catholics with clairvoyant abilities.

After the "Demon Nun" from The Conjuring 2 (2016) proved to be a popular horror antagonist, a spin-off focusing on the character was green-lit, making her the second character from the franchise to get her own feature after Annabelle (2014).

The events take place before Annabelle: Creation (2017), making it the first movie (chronologically) in the film series.

This was Valak's third appearance after The Conjuring 2 (2016) and Annabelle: Creation (2017).

An advertisement for the film was pulled from the video sharing site YouTube, as many users complained that the five second clip featuring a startling jumpscare was "too scary." Administrators thus removed the ad and apologized, claiming that it was not their intention to publish content that may potentially "offend" or "shock" viewers.

The name of the nun "Valak" can be seen on the playground where we first meet sister Irene on the playground equipment

Despite the Conjuring and its sequel being based on true events, this film is not based on any real historical events and is entirely fictional

The real name of the Nun - Valak - can be seen on the registration plate on the local villager's vehicle just before Father Burke, Sister Irene and Frenchie depart for the abbey.

Filming took place at the Corvin Castle in Hunedoara and in Sighisoara, Transylvania.

The film was originally set to hit cinemas in mid-July of 2018 but was pushed back closer to Halloween season on September 7, in hopes of doing as successful at the box office as It (2017) did last year, which smashed records and earned more than USD $700M.

Writer David Leslie Johnson-McGoldrick was replaced by Gary Dauberman and James Wan.

The film takes place in 1952.

Director Corin Hardy revealed that reshoots of the film were carried out by both him and James Wan. While Hardy reshot interiors, Wan was reshooting exteriors.

Principal photography for the film began on May 3, 2017 at Castel Film Studios in Bucharest, Romania, with Maxime Alexandre serving as cinematographer. Scenes were filmed in the Palace of the Parliament building in June, for a fee of EUR5,000 per hour.

The first movie in The Conjuring universe not to feature Joseph Bishara, either as an actor or composer.

Many of the nuns describe feeling the presence of spirits, the seeing of visions, and divination. This actually would never happen because according to traditional Roman Catholic religion, these things do not appear to prayerful individuals such as clergy

Every background for "church scenes" had to be done from scrap as it is banned in Romania to film actual churches.

It is a spin-off of 2016's The Conjuring 2 and the fifth installment in The Conjuring Universe.

At the end of The Nun, a scene from The Conjuring is shown where Ed & Lorraine are performing an excorcism on Maurice. This scene links the Maurice from that video as being the same Maurice from The Nun, but they edited the scene to fit into the narrative of The Nun: they edited in Jonas Bloquet into the excorcism scene, as he used to be played by a different actor; they added in dialogue of Ed saying "they called him Frenchie", so as to make a better connection.

The license plate on the vehicle that Sister Irene and Father Burke accidentally put their luggage on reads out "VALAK" which is a reference to the demon nun.

With the exception of the Abbess (who is revealed to be a corpse manipulated by Valak), the nuns are shown interacting only with Sister Irene, hinting at the revelation that she's been talking to ghosts all along.

The strategy to keep Jesuscrist's blood on the mouth and then spit it all over a demon's face has already been used in a movie. It was on Tales from the Crypt: Demon Knight (1995) where the female main character spits it the face of the main antagonist (a demon too).

Valak is mentioned by name only once, when Father Burke looks into the abbey's history. Everyone else refers to it as "the evil" or "the demon".

In The Conjuring 2 (2016), it is revealed that the "demon nun's" actual name is Valak.

The nun that Father Burke, Sister Irene and Frenchie bury would not have been buried in a Catholic ceremony because she committed suicide, which is a mortal sin.