When they show shots of any of the angels, they rarely blink. Nicolas Cage actually practiced not blinking and was able to do it for several minutes by the time the film was done. However, once he became human he was able to blink all he wanted.
When Seth falls to Earth appear pictures from his memory in black and white. It is because due his condition of angel, Seth can't see or distinguish the colors. It's a nod to Wings of Desire (1987), where the angels too were unable to see in colors.
During the opening sequence, you see 3 angels on top of a building as the camera pans up and around and you see the building's number, 444. 444 is a direct reference to the angelic realm: "Thousands of angels surround you at this moment, loving and supporting you. You have a very strong and clear connection with the angelic realm, and are an Earth angel yourself. You have nothing to fear - all is well."
When Meg Ryan is looking in the mirror and Nicolas Cage is not reflected, this was shot twice with motion control: once with Ryan and Cage, once without the actors, and mixed together. also, if you look closely enough, you can see a few strands of Ryan's hair morphing in and out where Cage's face was erased.
During the "falling" sequence near the end of the film, Seth's memories are shown. One of them is of a pregnant woman. This woman was portrayed by Elisabeth Shue, though you only see her stomach. Shue and Cage worked together in Leaving Las Vegas (1995).
This dialogue between Seth (Nicolas Cage) and Maggie - "Let's go." "Where?" "I don't care." "What will we do?" "Anything." - is virtually identical to an exchange between Cage and Deborah Foreman in Valley Girl (1983).
The concept of angels moving among the living, hearing their conversations, experiencing beauty that humans rarely stop to enjoy or even notice, yet not feeling any emotion or passing any judgment, was first discussed in the writings of Rainer Maria Rilke, specifically, his collection "The Duino Elegies".
When Seth becomes human, he can only understand the English language. This is the opposite to Wings of Desire (1987), where Damiel (Bruno Ganz) can speak any language after the fall to Earth and becoming human.
Although the movie show some important places of Los Angeles' city like Terminal Annex, Mulholland Drive, Los Angeles International Airport, Grand Central Market and Dodger Stadium, the library by where the angels walk is not located in Los Angeles, but San Francisco.
The sequence in the library with an old man reading a Hemingway's book is a nod to the Wings of Desire (1987). In it Curt Bois plays Homer, an aged poet that his poethical thoughts are read by Damiel (Bruno Ganz) some times.
The word "angel" is mentioned just two times throughout the movie, during a brief dialogue between Seth and Cassiel when both are walking by the beach to see the sunset. It's the only one time that they claim clearly their condition as angels.
Colm Feore's character is called Jordan Ferris. This is a combination of the last names of superhero Hal Jordan/Green Lantern and his love interest Carol Ferris. Eventually they were adapted into Green Lantern (2011), with Ryan Reynolds and Blake Lively playing Hal Jordan and Carol Ferris, respectively.
The trivia items below may give away important plot points.
The film shares many parallels and similarities to Hans Christian Andersen's 1837 short story, "The Little Mermaid." Both stories are about mystical beings who dream of becoming human after meeting and falling in love with one on earth. After repeated warnings from those around them, both protagonists find a way to become human and be with the one they love only to lose them shortly after.
Every death in the movie is shown through Seth's eyes. At the beginning of the movie, when Susan dies her soul appears at the side of Seth, talking briefly with him about life before both walk by a hospital corridor, in order to arrive Afterlife when the sunlight of a window shines to become Afterlife's white light. At the end of the movie, Seth assists dying Maggie in the road after her accident, but this time angels and white light not appear, due Seth now is a human being instead an angel. These events reveal death from two viewpoints, natural and supernatural.
During Nathaniel's feast in his home, his wife Teresa (Joanna Merlin) makes a photo to Cassiel and her granddaughter Hannah (Kristina Malota), who hugs Seth for the photo. When later Maggie looks the photo, she watches Hannah hugging a luminous white light where should be Seth's body, revealing his celestial nature.