J.J. Abrams conceived the idea for a monster film after visiting a toy store in Japan with his son while promoting Mission: Impossible III, wanting to create an American monster distinct from King Kong or Godzilla.

The severed head of the Statue of Liberty in the film was inspired by the poster for the 1981 film Escape from New York, which shows the statue's head lying on a New York street.

The project was secretly greenlit by Paramount Pictures in February 2007, with the title 'Cloverfield' changed several times during production to titles like Slusho and Cheese to avoid leaks, before reverting to Cloverfield.

The name Cloverfield comes from Cloverfield Blvd, the highway exit J.J. Abrams takes to his Santa Monica office, which used to lead to Santa Monica Airport, originally named Clover Field.

To keep the plot secret, actors were cast using scripts from Abrams's other productions like Alias and Lost instead of the actual Cloverfield script, with some audition scenes written solely for casting purposes and never used in the film.

Lizzy Caplan agreed to join the film without knowing its premise, purely because she was a fan of Lost, which was also produced by Abrams.

Filmmakers used a Panasonic HVX200 camera for most interior scenes and a Sony CineAlta F23 for nearly all New York exterior scenes to achieve the film's handheld, cinéma vérité look.

Several Los Angeles-area locations doubled for New York City, including an emptied Robinsons-May store in Arcadia standing in for Bloomingdale's, and a Warner Bros. backlot street used for the tank battle scenes.

Actor T.J. Miller, who played cameraman Hud, said he filmed about a third of the movie's footage himself, with roughly half of what he shot making it into the final cut.

After viewing a rough cut, Steven Spielberg suggested adding a hint about the monster's fate during the climax, leading to the addition of a countdown on the helicopter radio and air raid sirens signaling the 'Hammer Down' bombing.

Creature designer Neville Page based the monster's concept on the idea of an immature creature suffering from 'separation anxiety,' drawing comparison to frightened circus elephants that lash out.

The film has no traditional score except for Michael Giacchino's end-credits piece 'Roar! (Cloverfield Overture),' which director Matt Reeves confirmed was a deliberate tribute to Godzilla composer Akira Ifukube.

When the audio of the final post-credits whisper is played in reverse, it reveals the phrase 'It's still alive,' voiced uncredited by director Matt Reeves.

The film's shaky handheld camera style, nicknamed 'La Shakily Queasy-Cam' by Roger Ebert, caused motion sickness in some viewers, prompting theaters like AMC to post warnings about the filming style.