Imagine E.T. recast as a low-rent comedy, conceived by the cheerfully profane Shaun of the Dead co-stars Simon Pegg and Nick Frost, and you should have some idea what to expect from Paul, their first big-screen collaboration as screenwriters.

Absent the subversive touch of Edgar Wright, who directed and co-wrote Shaun (2004) and the pair’s buddy-cop parody Hot Fuzz (2007), Paul is a slighter diversion, as much a loving homage to geek culture as a gentle send-up of the genre’s lesser entries, including 1988’s Mac and Me.

Perhaps part of the joke is that Paul, a little green man from a galaxy far, far away, is so physically unimpressive. In the midst of a CGI revolution that has allowed filmmakers working on even the tiniest budgets to create credible-looking close-encounters fantasies, Greg Mottola’s laidback farce is the anti-spectacle.

Not that it really matters. Nobody going to see Paul will be expecting anything more than what the movie so generously offers: endearingly juvenile jokes about alien probes, comic-book nerds and slack-jawed yokels.

Pegg and Frost are credited with the script, but some of the movie’s sharpest dialogue consists of the pair’s teasing exchanges, which have the feel of spontaneous banter. The two play off each other – and Seth Rogen, who lends Paul his sardonic deadpan – in a way that seems both inspired and utterly natural.

Yet their handling of a stellar supporting cast – including Bill Hader, Kristen Wiig, Jason Bateman and Alien star Sigourney Weaver – leaves something to be desired.

Bateman, a gifted comedian, is stuck in straight-man purgatory, playing a government hard-ass bent on crashing Paul’s rendezvous with his mother ship. Wiig, as a recovering creationist who celebrates her liberation from fundamentalism by cursing a lot, is impressively exuberant, but her one-note role grows increasingly tiresome.

Your reaction to Paul may depend to some extent on your appreciation of its winking references to sci-fi classics including Predator, Star Wars and Back to the Future. Even Steven Spielberg, the Close Encounters man himself, phones in a cameo –literally.

Beyond that, the movie coasts somewhat lazily on the considerable charm of its stars. For Rogen, whose smart-alecky E.T. is a hard-partying pothead, Paul is no stretch. And, frankly, we’ve come to expect better from Pegg, Frost and Mottola (Adventureland), whose résumés suggest collaboration magic that, in the end, the movie delivers too sporadically.