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.
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.
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".
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.
Sigourney Weaver jumped at the chance to appear in the film as she felt it was a love letter to science fiction fans and a genre that has been very good to her in her career. Simon Pegg himself even had a crush on Sigourney and even wrote a poem about her at Bristol University. He would get his wish when he would work her in one scene in this movie.
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.
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.
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).
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.
During the fight scene in the woods at the end of the film, before punching Sigourney Weaver to save Kristen Wiig, Blythe Danner says "get away from her you bitch!". This is the same dialogue spoken by Weaver's character Ripley in Aliens when defending Newt from the alien queen.
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: Episode IV - A New Hope.
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.
Both Seth Rogen (Paul) and Jeffrey Tambor (Adam Shadowchild) both play George Sr. Bluth (Rogen playing a young George Sr. in flashbacks) in season 4 of Arrested Development. George Sr.'s wife, Lucille Bluth, is played in the flashbacks by Kristen Wiig, who plays Ruth 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 1.
5 of the cast members have also made appearances on Arrested Development as part of the main cast (Jason Bateman, Jeffrey Tambor) and the others in guest roles (Kristen Wiig, Seth Rogen and Jane Lynch.)
As Graeme and Clive are looking at the firework mortars, a commercial entrance alert (door bell) is heard playing the "Close encounters" five tones. And the mortar they selected is called "The five tones".
When The Big Guy arrives to arrest Paul and his companions he calls him Mork, which is a reference to Mork and Mindy, a sitcom in the 70's about an extraterrestrial(Robin Williams) living with a human woman(Pam Dawber)
There are many pop culture, sci-fi, 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), the "Empire Strikes Back", "Invincible" (by Robert Kirkman), and "Eightball" (by Daniel Clowes). They attend the San Diego Comic Convention twice (with many comic & sci-fi references). Other references include a lot of Steven Spielberg projects (E.T., Close Encounters, Temple of Doom, etc.) Steven does lend his voice to one scene. Other homages include: the "Alien" series (Sigourney Weaver), "Predator", "Back to the Future", "X-Files", "Star Trek" (Simon Pegg stars in the recent reboot), and "Star Wars". Supporting cast member, Bill Hader, is a huge comic book fan. He teamed up with SNL 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 Sci-Fi (Master Yoda).
In the roadhouse, when the fight starts, the redneck says "oh no, sailors!" This is a reference to a line in David Bowie's song "Life on Mars" ("take a look at those sailors fighting in the dancehall").
Actor John Carroll Lynch has a line, "I'm on a mission from God!" which is similar to the line from 'The Blues Brothers' ("We're on a mission from God!") directed by John Landis ('American Werewolf in London', 'Three Amigos!', and 'Coming to America'), among others.
I've never seen this listed anywhere else, but when they are at the gas station supposedly in Ely, Nevada) 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 R.V.
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.
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.
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.
During the end credits a burn scarred O'Reilly (Joe Lo Truglio) is briefly seen and heard shouting "I know those guys!" showing that he actually survived the exploding farm house that occurred towards the end of the film.
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. The other movie on the marque is Steven Spielberg's Duel (1971).