Simon Pegg and Nick Frost came up with the concept for Paul while waiting out a rainstorm on the set of Shaun of the Dead (2004). They handed producer Nira Park a sketch drawing of Paul wearing an FBI T-shirt, flipping-off the viewer, with a caption that read, "In the U.S., everybody is an alien." The actual drawing can be seen during the closing credits.
For much of the filming, Seth Rogen was off filming The Green Hornet (2011) and so was unable to completely inhabit the character Paul's motion and interact with the other actors. Joe Lo Truglio, who also plays O'Reilly in the film, stood in and finished what Rogen didn't complete. He studied Rogen extensively in order to impersonate his voice, performed on his knees to capture Paul's physical presence and even improvised in character as Paul. When filming wrapped, Rogen came in and provided the character's voice.
While researching the film, Simon Pegg and Nick Frost actually drove an RV along the route Clive and Graeme would take in the script. During the trip they stopped at the actual Little A'le'inn, where they encountered a chatty waitress and some belligerent locals. The encounter inspired them to include it in the script.
At the gas station when they're being asked about the lack of guns in the UK, the main characters confirm that no-one carries guns "except for farmers". This is a reference to the actors' previous film Hot Fuzz (2007), where "this is the country, everyone and their mums have guns"', "Like who?", "Farmers", "Who else?", "Farmers' mums".
At the 2010 San Diego Comic Con, Seth Rogen admitted that he had to consult help from actor Andy Serkis for inspiration in order to get the idea on how to use the motion capture suit for his CGI character Paul. Serkis had originally portrayed Gollum (through motion capture and voice) in the Lord of the Rings films.
When O'Reilly shoots out at the RV from the farm house causing it to explode, the camera zooms in really quickly in a type of shot called a "dolly zoom". This was an intentional homage to an identical shot from Jaws (1975) where Sheriff Brody blows up the shark.
Simon Pegg is the Godfather to co-star Blythe Danner's granddaughter, and Gwyneth Paltrow's daughter, Apple Martin (Danner is Paltrow's mother). This marks the first film together between Danner and Pegg.
In the town where the comic book store shoot-out occurs, the movie theater marquee is announcing a double feature including Easy Rider (1969). The scene was filmed in Las Vegas, New Mexico, a block down the street from where Easy Rider was actually filmed.
During the sequence in the comic shop, an issue of "The Boys" (a genuine series) can be seen on one of the racks. One of this comic's main characters (Hughie Campbell) was intentionally drawn to look like Simon Pegg. Pegg wrote the introduction for the first collected edition of "The Boys".
In an interview, Simon Pegg and Nick Frost mentioned that making a big budget studio film (they cited Paul as being more expensive than Shaun of the Dead and Hot Fuzz put together), there was a need for compromise: not in a negative sense, but that the studio also had a say in the filmmaking process. For instance, they originally felt an older actor like Jack Nicholson or Rip Torn would be ideal for the voice of the title character, but when the studio suggested Seth Rogen they liked the idea, due to Rogen's distinguishable voice.
At the beginning of the film Graeme and Clive state that their itinerary will eventually take them to New Mexico. Their characters never actually get to New Mexico although, ironically, the majority of the movie was filmed there.
When a running gag question is asked at various times, "Who the hell is Adam Shadowchild?", the answers are; Planet Fall Trilogy; The Venusian Pangenesis; Jenny Starpepper and the Great Brass Hen; The Robot's Mistress; The Jupiter Praxis; Jenny Starpepper and the Huge White Gibbon; Night of the Moths; Prison Hulk 441; Jenny Starpepper and the Spitting Worm; and Fluxing Uranus.
The over-priced sword that Clive looks at during Comic-Con and then eventually buys in Wyoming was intended as a nod to Blade (1998), but the filmmakers couldn't get permission to use the reference, so they simply called it "The Black Vampire" realizing the audience would get the joke.
The trivia items below may give away important plot points.
The movie begins in 1947 in Moorcroft, Wyoming, where Paul crashes to Earth. In Close Encounters of the Third Kind (1977), Roy Neary (played by Richard Dreyfuss) travels to Moorcroft looking for an answer about his meeting with an UFO. The mountain where Paul signals his ship, called Devil's Tower, is also used in Close Encounters.