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 F.B.I. 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.
While researching the film, Simon Pegg and Nick Frost drove an RV along the route Clive and Graeme take in the script. During the trip, they stopped at the Little A'le'inn, where they encountered a chatty waitress and some belligerent locals. The encounter inspired them to include it in the script.
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 (2004) and Hot Fuzz (2007) 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 2010 San Diego Comic Con, Seth Rogen admitted that he had to consult help from 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 Hobbit and The Lord of the Rings films.
For much of the filming, Seth Rogen was off filming The Green Hornet (2011), and so was unable to completely inhabit 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.
The scene where Agent Zoil (Jason Bateman) is in the car, and is talking to The Big Guy (Sigourney Weaver) on the radio, and he ends the call by shooting the radio with his gun and exclaiming "boring conversation anyway", is a direct reference to Star Wars (1977).
At the movie's beginning, Graeme and Clive state that their itinerary will eventually take them to New Mexico. They never actually get to New Mexico although, ironically, the majority of the movie was filmed there.
The odd presence of sailors in the bar fight in landlocked Wyoming is actually a direct reference to a line in David Bowie's song "Life on Mars" ("Take a look at those sailors fighting in the dancehall; ooh man, look at those cavemen go"). Alternatively it is both an in-joke about the presence of sailors in seemingly all Hollywood bar fights, and a nod to Steven Spielberg's 1941 (1979).
In the scene where Paul is talking to Steven Spielberg on the phone, Paul is calling from a huge room that looks like the warehouse where the Ark of the Covenant was stored at the ending of Raiders of the Lost Ark (1981).
Seth Rogen (Paul) and Jeffrey Tambor (Adam Shadowchild) both play George Bluth Sr. (Rogen playing a young George Sr. in flashbacks) in season four of Arrested Development (2003). George Sr.'s wife, Lucille Bluth, is played in the flashbacks by Kristen Wiig, who plays Ruth Buggs in this movie. In addition, Jason Bateman (Agent Zoil) plays Michael Bluth throughout the series, and Jane Lynch (Pat Stevens) plays Cindi Lightballoon in season one.
When Agent Haggard is chasing Paul near the movie's ending, he tries to shoot at the RV. He fires a shot at him as he says "Smile, you son of a bitch." This is another Steven Spielberg move, in this case Jaws (1975). Where Roy Scheider's character Chief Brody says the same thing right before he shoots the air tank in the shark's mouth.
When the 'running gag' question is asked at various times; "Who the hell is Adam Shadowchild?", the answers (referencing his publications) include; 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.
When The Big Guy arrives to arrest Paul and his companions, she calls him Mork, which is a reference to Mork & Mindy (1978), a sitcom about an extraterrestrial (Robin Williams) living with a human woman (Pam Dawber).
As Graeme and Clive are looking at the firework mortars, a commercial entrance alert (door bell) is heard playing the five tones from Close Encounters of the Third Kind (1977), and the mortar they select is called "The five tones".
After Paul heals Ruth's eye and she wakes up from her nap, the Jesus shooting Darwin "EVOLVE THIS" cartoon is no longer on her T-shirt, suggesting that she was healed in more ways than one; having also been freed from her religion.
There are many popular culture, science fiction, and comic book references in this movie. During the movie, the two main characters wear T-shirts featuring: Flash from DC Comics, Ming the Merciless (Flash Gordon), Star Wars: Episode V - The Empire Strikes Back (1980), "Invincible" (by Robert Kirkman), and "Eightball" (by Daniel Clowes). They attend the San Diego Comic Convention twice (with many comic and science fiction references). Other references include a lot of Steven Spielberg projects (E.T. the Extra-Terrestrial (1982), Close Encounters of the Third Kind (1977), Indiana Jones and the Temple of Doom (1984), et cetera.) Steven does lend his voice to one scene. Other homages include: the Alien franchise (Sigourney Weaver), Predator (1987), Back to the Future (1985), The X-Files (1993), Star Trek (Simon Pegg stars in the recent movies), and Star Wars. Supporting cast member, Bill Hader, is a huge comic book fan. He teamed up with Saturday Night Live (1975) cast member Seth Meyers and Kevin Maguire for a one-shot Spider-Man comic book. Even the music soundtrack has many inside references, such as John Williams, The B-52's, and David Bowie. One main scene, takes place in a comic book store with tons of references to comics (Hellboy) and science fiction (Master Yoda).
I've never seen this listed anywhere else, but when they are at the gas station (supposedly in Ely, Nevada, but really on State Road 44 between Farmington and Albuquqerque, New Mexico) after the State Trooper nods at Clive, you can see the word FIRST drawn on the side view mirror of the RV.
The motorhome is clearly far too large for the two of them, perhaps being a 6 or 10 person Winnebago. Such vehicles are typically $250/night to hire. That said, $125/night/person for the full USA road trip experience may well have been within their budget, especially as they may have expected to pick up guests along the way. Note the quality of smaller motor homes drops dramatically for smaller hire vehicles and would not therefore deliver the full experience of a lifetime. Perhaps they are keeping their options open to meet people, as part of the vacation.
The trivia items below may give away important plot points.
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.
During the fight scene in the woods, at the movie's ending, before punching Sigourney Weaver to save Kristen Wiig, Blythe Danner says "Get away from her you bitch!" This is the same line spoken by Weaver's character Ellen Ripley in Aliens (1986), when defending Newt from the Alien Queen.
The movie begins in 1947 in Moorcroft, Wyoming, where Paul crashes to Earth. In Close Encounters of the Third Kind (1977), Roy Neary (Richard Dreyfuss) travels to Moorcroft looking for an answer about his meeting with a U.F.O. The mountain where Paul signals his ship, Devil's Tower, is also in Close Encounters of the Third Kind (1977).
During the ending credits, a burn scarred O'Reilly (Joe Lo Truglio) is briefly seen and heard shouting "I know those guys!", showing that he survived the exploding farmhouse that occurred towards the film's ending.
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 Chief Brody blows up the shark. There was another nod to Jaws when Agent Haggard pulls along side the RV and aims his gun at Paul and says, "Smile you son of a bitch!". This was Chief Brody's final line in Jaws right before he blew up the shark.
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 (1969) was filmed. The other movie on the marquee is Duel (1971).
After Paul heals Ruth and she excuses herself to freshen up, she wears her "Evolve This" shirt inside out. Cloth is heard ripping in the background, and the shirt has gone from a crew neck to a v-neck.