Writer/director Joe Cornish was inspired to make this film after actually being mugged in real-life one night (much in the same way Sam was as portrayed in the film). He noticed his five young assailants were as scared as he was, and started researching their lives.
The film-makers only used CG effects when absolutely necessary, and to enhance practical effects for the creatures rather than replace them completely. Even the smaller female alien that appears before the credits was a petite woman in a creature suit. A puppet-type head was used for some of the attack shots wherein Moses is suddenly bitten. The creature's head was a carefully constructed mask that had no eyes, and even the glowing mouthful of large, carnivorous teeth were achieved by animatronics (including twelve "servos") rather than added in post. The film-makers admit that it did help save money, but also had an unexpected benefit. The actors, rather than reacting to something that wasn't there, admitted that they were genuinely and unexpectedly frightened by the look and movements of creatures actually present (especially during chase sequences when a creature/creatures would pursue them at full speed). Nearly every actor said they felt especially intimidated-- many surprisingly so-- by the physical presence in a way they would not have if the creatures had been added digitally later. The same went for the majority of the settings; the director said it added authenticity and atmosphere to shoot on a set rather in front of a green screen.
Members of the gang compare the film's aliens to various fantastical creatures, all British in origin, namely: Dobby the house-elf from J.K. Rowling's series of Harry Potter novels; Gollum from the works of J.R.R. Tolkien; and Gremlins, who while they are now best-known for the two American films by Joe Dante, were born out of the imagination of RAF fighter pilots during World War II, and were initially popularized by author Roald Dahl in his first novel.