I was curious about Geoff's construction when I had the opportunity the other day to pick up a couple of model skulls at a salvage outlet for $2 each thinking about using them in the future for Halloween decorations. It occurred to me, hey I could build a "Geoff Peterson", if only a 'talking' head for the old computer cubicle.
Since this was the one thread that even mentions building a "Geoff" I thought I would at least put down a couple of things I discovered. What I bought were
Linberg models which on reflection don't quite look much like Geoff which interested me in finding out what skull/skeleton Grant Imahara probably used to construct Geoff.
I did find
this skull which I think is referred to as a "Bucky" (probably because the skull's teeth look like they came from a buck-toothed person) I'm attaching a photos which to my eye show Grant building Geoff who seems to have all the screw holes where springs for the jaw and skull clasps have been removed to modify it for animation. I'd love to know if the eyes are some off-the-shelf LED light housings, plans seem to refer to them a "12V indicator LEDs". I suppose if one surfs the web long enough or pokes around auto parts stores one might run across a match or close substitute.
Some links via
Popular Mechanics, complete
"Bucky" skeleton
Though I'm attaching the skull pic only here, it's also available as a full skeleton which if you Google around you'll happen upon full length shots of Geoff which show he's on a 4 caster black rolling stand that these "Bucky" skeletons come with. The full skeleton is more money than I'd want to spend, but just using the skulls and say two cheaper skeleton hands along with cheaper material like PVC or electrical conduit to create the body it might be a fun, cheap build.
The electronics, motors and all are a bit sketchy on details of how Geoff is animated, but since there are a ton of how-tos videos to animate skulls and skeletons on YouTube a knock-off Geoff might be fun to build. It would be great if someone does make one if they can provide a tutorial on how to make or get all the servos and stuff one would need like me starting from dead scratch...
..especially if you have his 7 or so phrases included 
