When it came to depicting the eponymous alien from Paul, actor/co-writer Simon Pegg was very clear about what he wanted: the iconic gray creature with an enormous head, big eyes and spindly limbs. In other words, very familiar with hints of Close Encounters of the Third Kind and E.T., which was fitting since Steven Spielberg makes a hilarious cameo on the phone with Paul.

"Right off the bat, Simon envisioned it as a turn away effect so Paul wouldn't stand out," remarks Anders Beer (Hellboy II: The Golden Army), the animation supervisor from Double Negative, who collaborated with Jody Johnson, the visual effects supervisor. "He didn't want him romanticized in any way; he didn't want him to be a cartoon or a creature effect. He said many times that it cannot be Jar Jar Binks."

They eventually arrived at a design they liked, which was sculpted by Spectral Motion. But given that this was an indie buddy comedy about two geeky Brits (Pegg and Nick Frost) and their close encounters with an iconoclastic alien (voiced by Seth Rogen) outside Area 51, the options were limited.

"We had to make some compromises with the design, specifically the eyes," Beer continues. "They wanted a dark, penetrating look, but it was too creepy. It was hard to connect with it and make eye lines work. We went with eyes that are more naturalistic: they've got an iris and a pupil.

"Originally they were thinking of using Bill Hader for the voice and we did a great test with Hader, but then they decided to go with Seth Rogan, who is the physical antithesis of Paul. The initial test with Rogen was very preliminary with more jowly features and matched more closely to what Rogen sounded like. They pushed it more in the familiar gray alien. Actually, the biggest challenge was coming up with a performance in the alien that didn't do away with Rogen's charm and charisma but still embraced the physical uniqueness of Paul's design. The primary way was to do it with animators but not make the performance too broad."

Double Negative, therefore, came up with an appropriate animation pipeline for the 300 shots, using a team of about 30 animators. "What I came up with was a brain trust with a couple of leads and the head of animation at Double Negative [Eamonn Butler]," Beer says. "It revolved around reference. We had Nick and Simon and a rough geography laid out on a soundstage floor in Culver City and we had Seth in a Xsen Moven suit [based on inertial sensors]; and we had three high-def witness cameras. We basically rehearsed the whole film for about four days on a stage before principal photography. It was a great starting point to calibrate ourselves to Seth Rogen. It was also a great start for the editorial side because they didn't have the budget to do full-CG previs.

"In the end, we didn't use any of the MoCap data. We were still steered for the very specific design. It was a low-budget film about shooting in a traditional sense and the weather affected everything. I was on location with the crew for three months in Santa Fe and other locations in New Mexico, shooting for a character that wasn't on set. We had puppets and other stand-ins (a bunch of titanium rods with adjusters and ping-pong balls). We shot clean passes at everything, and, when the actors interacted with Paul, we used things that were minimally invasive and cleaned up the plate later.