Exploring Valve Archive, part 14. valvearchive.com>archive>Portal>Portal 2>Other>Reference Photos> combineoverwiki.net>wiki>Caroline> Did you know: after casting calls failed to find the perfect face for Cave and Caroline, Valve would turn back to their own staff for inspiration? Pictured above, Cave's likeness is actually based upon Bill Fletcher, Valve's lead animator for Portal 2 and other projects. Similarly, Caroline's is based upon Laura Dubuk, an independent contractor to Valve for environment art and creation. The two would would later cosplay as their counterparts at Comic-Con 2012, where they took photos with fans and other Valve employees (the one holding the Wheatley plush above is Karen Prell, lead animator for Wheatley and other Portal 2 robots).
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Here's a little known fact about Laura Dubuk. At one point during Half-Life 2's development, her likeness was actually considered to be used for one of the female citizens, but her model was for whatever reason cut. We've still got access to the original face map thanks to 2016 leaks.
Why does Bill look like an older version of Linus from LTY
*LTT
He looks like Linus Tech Tips