Hey, It's That Guy!: Michelle Monaghan has a blink-and-you'll miss it role as one of the demons. She's the one who says, "Holy water...?" before they're all killed by the fire sprinklers. Her role was intended to be larger, serving as Constantine's demon lover, but her part was cut from the final film.
A lot of fans of the film (and comic) consider the editing out of Constantine's half-demon sex partner Ellie, based on the succubus Ellie from Hellblazer and played by Michelle Monaghan, to be a loss for the film. The reason given was that it would be harder to believe that Constantine was a bitter loner if he was sleeping with a gorgeous woman, even if she was evil.
Also, the deletion of many scenes from the original script which dealt with Chas' character. Even if you aren't fond of Shia LaBeouf, the original script gave his character something to actually do, whereas in the final film he disappears for a good two-thirds during the middle, which begs the question why his character wasn't also cut.
Actor Allusion: "Danse Voudou" finds Chas in Louisiana investigating occult murders done with a blade. He even takes his Yellow Crown Cab Co. car with him.
Blooper: In the final scene of the TV series pilot, the level of beer in John's glass changes from full to having a large sip taken out of it and back to full.
The Danza: If you squint. "Chas" is usually short for "Charles," and he's played by Charles Halford.
Liv staying for the actual run instead of being replaced after the pilot.
In early production, the TV series was supposed to have Constantine based in New York City, as promotional materials have Midtown Manhattan in the background, and the Ravenscar Asylum also in the States; however, the final product has Constantine based in Atlanta and Ravenscar back in Northern England.