We've already received a sneak peak at what to expect from Portal 2, in the form of the recently unveiled achievement list, but a number of story details still remain a mystery.

While Valve has already revealed that fan-favorite killer AI GLaDOS and protagonist Chell will return for the sequel, the development team has been keeping a lot of Portal 2 story-details under wraps.

Many details are still under tight wraps, but PAX visitors got an early glimpse at the opening moments of Portal 2 - including the introduction of Wheatley, the personality sphere voiced by Stephen Merchant as well as the reveal of a new character, Cave Johnson, the founder of Aperture Science voiced by J.K. Simmons (J. Jonah Jameson in Sam Raimi's Spider-Man films).

The new footage showcases Valve's interest in improving story-telling in Portal 2. Sure the story in the original Portal was engaging and kept players chuckling as they were drawn farther and farther into the story - however, aside from the homicidal robot GLaDOS, there weren't any other characters to interact with. As a result, the first game was a somewhat isolated journey about exploring and discovering the world of Aperture Science - and subsequently short on interaction with other self-aware characters.

To help immerse the player in Portal 2 - as well as add a thick does of humor (not to mention teach gamers the basic game mechanics) Valve has created Wheatley - who is one of the GLaDOS personality cores that has learned to act independently. In the opening minutes of the game, Wheatley awakens Chell from her sleep - and helps her escape the crumbling confines of her quarters. The one-sided Wheatley banter (against Valve's traditional silent protagonist) hits the same quirky tone as GLaDOS from the first title - and helps the player get up to speed on the events between the first and second game.

Portal 2 Video Preview Dialogue

Portal-fans who have been following the game's development closely will probably remember hearing about Wheatley; however, Valve's latest reveal came in the form of a disembodied introduction to Aperture Science founder and CEO, Cave Johnson. While the character was not shown on-screen, relying on PA announcements, his presence is almost larger-than-life - with a number of hilarious one-liners that hint at much larger, and infinitely more bizarre, experiments (than just portal guns) at Aperture Science. I won't spoil them here but, for the record, one topic may or may not involve praying mantis/human hybrids.

Stay tuned for a full interview with the Valve writers - regarding their approach to developing characters, story, as well as blending the worlds of Portal and Half Life.

Follow us on Twitter @benkendrick and @gamerant and let us know what excites you most about Portal 2.

Portal 2 will be opened, I mean released, for  Xbox 360, PS3, PC, and Mac on April 20, 2011.