Dipper? Or Bill Cipher?
For all the devoted fans of Gravity Falls, Dipper's behavior seemed a little off recently. For example, remember when Uncle Stan tried to open a portal to get back his twin brother? Dipper was yelling at Mabel to shut down the portal and not trust Stan. Does that sound like him? Not to mention the way his pupils elongated when the portal was turned on.
But let's back it up to the beginning. When Stanford calls Stanley over to help him keep his journal hidden in the Gravity Falls episode when Stanley is reminiscing about his childhood, Stanford first shines a flashlight into Stanley's eyes before letting him in the door for an unknown reason. When Stanley asks what Stanford's doing, he explains, "I'm checking for- oh never mind." Maybe he was checking Stanley's pupils to see if he was possessed? This would explain how Bill could possess Dipper without Dipper having long and elongated pupils. Dipper's pupils would only be elongated when exposed to light, such as a flashlight. The light from the portal was probably what caused Dipper's pupils to turn rod-shaped in the second picture.
I'm sure you all remember the first time Dipper was possessed by Bill Cipher. Well let's fast forward now, to where I believe Dipper got possessed the second time. Do you remember the Shapeshifter? No? Does this picture ring a bell?
This is the picture the Shapeshifter showed Dipper when he said that Dipper would die in this pose. And at Pacifica's mansion Dipper posed in an awfully similar way...
Could it be that when Dipper was in that form at Pacifica's mansion, he died, and the Dipper in the following episodes was just Bill Cipher controlling his body? Sounds creepy right? There's more. In Pacifica's mansion there's a banner that looks like this:
Was this banner foreshadowing the future of Gravity Falls? Don't forget what Stanford's college buddy, Fiddleford, said when he went into Stanford's portal and then got pulled out, "When Gravity Falls and the Earth become sky, beware the beast with just one eye." Could the beast with just one eye be Bill Cipher? And could the burning tree on the banner represent Dipper, who Bill Cipher always calls Pine Tree? And for the last piece of evidence, in the episode of Gravity Falls titled the Stanchurian Candidate, when Stanley is running for mayor, Dipper and Mabel are controlling him with a special tie designed by Stanford. Maybe this is foreshadowing how Bill is controlling Dipper? Finally, in this same episode, Lil Gideon possessed his father, Bud Gleeful, so Bud could win the spot of mayor. Could Gideon have cashed in on the favor Bill Cipher owed him and asked Bill to possess his father? Also, the face Bud Gleeful makes when he gets possessed is pretty familiar...
And his shadow is similar too..
Both of his arms are outstretched like Dipper's in the shadowy picture (sorry it got a little cut off). Do you believe my theory?