Far be it from us to spoil even one second of Portal 2 for those who haven't finished it, but for those who have, Rock, Paper, Shotgun has posted a great interview with Valve's Chet Faliszek and Jay Pinkerton, two of the game's writers. In addition to discussing some of the plot elements of Portal 2, Pinkerton and Faliszek also shed some light on the early design stages of the game and also what it was like to work with J.K. Simmons and Stephen Merchant, who voiced Aperture Science founder Cave Johnson and Wheatley the AI core, respectively.
And, of course, there's talk about GLaDOS and how she has evolved throughout the series. Finally, Faliszek and Pinkerton chat about the open culture of Valve, which encourages creative experimentation (much of which finds its way into Valve's games). If you've wrapped up Portal 2, be sure to give the twopart interview a read.