Constantine's line in which he says that he doesn't want Angela to be another ghost following him around is a reference to the comics, where he is haunted by the ghosts of friends who have died because of him.
The character of John Constantine was originally created by Alan Moore, during his run on DC Comics' "Swamp Thing". However, following his negative experience with From Hell (2001) and The League of Extraordinary Gentlemen (2003), Moore decided to reject all money and credit from Hollywood on any adaptations of his work. Thus, he gave all the money he would've gotten to the artist who drew the character with him, and rejected his own "created by" credit from the film.
The tattoo on Constantine's arms is the alchemical symbol of the Red King, which was worn (by him) for protection. It is "the fire triangle with three radiating arrows below represents the 'Perfect Red King,' the Sulfur of the Philosophers."
The original title, Hellblazer, was changed because it was too similar to Hellboy (2004). The movies were scheduled to be released within a short space of time between them and it was decided that having such similar titles would hurt the sales of tickets. Therefore Hellblazer was changed to Constantine.
Originally, Tarsem Singh was attached to direct with Nicolas Cage to star. However, the director said that "with Cage, I cannot make the film I had wanted to". Soon after Singh left the picture, Cage did as well.
In the comic book "Hellblazer" on which the movie is based, the character of John Constantine is from Liverpool, and is drawn to look like the singer Sting. In the movie, the character is from Los Angeles. It is widely believed that the change was made specifically for Keanu Reeves. The truth is the original script retained his nationality but got no response from producers. So the writer changed him to an American and the script quickly gained attention.
In the scene where John meets Hennesy for the first time there is a TV with a cartoon playing. At the end of the conversation as John goes to exorcise the demon from the little girl, he leaves the room and shuts the door. The cartoon in the movie shows a cloud of smoke shutting a door at the exact same time as John leaves and shuts the door.
Aside from John Constantine, the only characters to survive the transition from comic to movie are Ellie (a demoness mostly cut from the original theatrical release), Papa Midnite, Chaz Chandler and Gabriel. All of the other characters were invented for the movie.
Ravenscar Hospital is named for a mental institution that features in the Hellblazer comic book series. John Constantine spent four years in and out of Ravenscar after he had a nervous breakdown due to a botched exorcism.
In the lower right-hand corner of Isabel's toe tag, there's the number 616. Whilst some people interpret the Mark of the Beast to be 666, others interpret it to be 616, hence its appearance on the toe tag.
Shia LaBeouf was recommended to producer Akiva Goldsman for the role of Chaz by Will Smith following his impressive performance in I, Robot (2004). The three of them worked in that film, with Smith and Goldsman as producers.
In a deleted scene Angela is looking over Constantine's arrest and booking report, it lists his date of birth to be 09.02.66, or September 2, 1966. Additionally, it lists his height at 6-1 (or six feet, one inches), weight of 180, hair color as Blk (Black), eyes as Brn (Brown), Complexion as Fair, marital status of Single, and U.S. citizenship. Report details that he was pulled over for a broken tail-light and upon search of the car gun parts were found along with a Bible, Catholic cross, and some type of water bottle. Ultimately it was determined there was "insufficient evidence to prosecute".
On the side walls of Beeman's bowling alley there is a logo of three balls each having a line that juts off from them, looking more like  than anything. This can be seen at the beginning of the scene where Beeman discovers the truth to the demons trying to pass over.
The score credit in the film was originally only Brian Tyler, but Klaus Badelt was brought in by producers late in post-production to help enhance the comedic aspects of the film. Tyler and Badelt share a dual credit in composing the score.
Constantine slits his wrists, starting with his left, and Lucifer reaches out to claim him with his left hand, in keeping with the traditional depiction of the left ("sinister") side being associated with evil.
In the original script Father Hennessy was supposed to die of overeating and get thinner the more he eats. The scripted death was inspired by a character in the first issue of Hellblazer who died the same way.
Ellie, a demon who was Constantine's lover, was almost completely cut from the film. Director Francis Lawrence stated that she was left on the cutting room floor because he wanted Constantine to be completely alone when he meets Angela. Ellie was played by Michelle Monaghan. Her scenes can be seen in the deleted scenes section of the DVD. In the theatrical cut, she can be seen briefly in the hospital scene; when Constantine sets off the sprinkler system, she's the one who exclaims, "Holy Water!"
There is a post credit scene in the movie: John visits Chas' grave, complimenting his sacrifice. As he turns and walks away, Chas appears perched over his own tombstone, showing his new white wings. He then flies away.
The character of Father Hennessy is a mixture of three of Constantine's allies from the comic book. They are Nige Archer who was a psychic, Rick the Vic who was a priest, and Header who was a portly substance abuser. And, like Hennessy, all are killed for being associated with Constantine.
Special features on the dvd reveals that it wasn't really Rachel Weisz in the scene where her character flies backwards through the building. A small replica of her body was made which then was "shot" backwards through a miniature office, adding light and wind affects.
In the scene of the last fight between Mammon, Gabriel and Lucifer when Constantine cuts his wrist his watch stops around 17:20. When Lucifer leaves in the end the clock in the background reads 17:20, which means that the final battle takes no more than a minute in the world despite the scene taking up much more time.