I heard, actually, that Smith made his own bed on that one. They wanted him to do something "cooler" with Guy, and Smith makes up the half-alien shapeshifting crap on a total bullshit whim, like, "Jesus this is so stupid they'll laugh this off and realize how dumb it sounds and let me just go ahead with my original pitch," and instead they go "great, awesome, go with it." And Smith is left holding the bag and blinking and going, "wait, really? Aw, fuck."