SPOILER WARNING! This interview is about the plot, characters and script of Portal 2, so it necessarily contains spoilers right through to the game's ending. You should wait until you've completed the game to read it (which is why we've held publication back until today).

Erik Wolpaw looks tired. The mousy, bespectacled writer has a cracked voice and a bad cold. He seems nonplussed because, at the time of this interview in Valve's offices overlooking a typically drizzly Bellevue, Washington, he finished work on Portal 2 four weeks ago, yet there's still four weeks to go until its release. He's in limbo.

Hopefully he feels better now, because his game is out in the wild, being loved by players and showered in praise by critics  including me, in our 10/10 Portal 2 review.

As its scriptwriter, along with National Lampoon veteran Jay Pinkerton and his former collaborator on the Old Man Murray website, Chet Faliszek, Wolpaw's contribution to its brilliance and success is not small. "It's both shameless and devastatingly successful in its pursuit of belly laughs," I wrote. "Portal was a sequence of great jokes, but Portal 2 is that rare beast, an actual video game comedy  and one of the funniest ever."

It's an even greater achievement when you consider that Portal 2 manages this with no 'actors' on the screen. Wolpaw and Pinkerton (Faliszek came on later and mostly contributed lines to the co-op campaign) successfully conjure characters from voices that are either idiotic, psychopathic machines or recorded ghosts from the distant past  and in doing so, they expand the icy skit of the original Portal into both a human drama and a compelling action yarn.

So I'm eager to ask Wolpaw (who, before joining Valve as a writer, contributed to the script for Double Fine's brilliant Psychonauts) how they pulled it off. Remember: spoilers!