Okay, so I don't know if anyone's talked about this yet, but while watching Gravity Falls I noticed something about the title sequence. The first few times I saw the part where Dipper is pointing out that huge footprint to Grunkle Stan I merely thought that the three Pines were going on a monster hunt and were tracking some sort of monster. You know, family fun time. When it became evident that Grunkle Stan 'didn't believe' about the supernatural stuff the scene became rather sad, as I then realised Dipper was trying his best to prove the supernaturals' existence to Grunkle Stan. The poor little noodle would probably take Grunkle Stan out on one of his and Mabel's explorations in the woods to prove that he wasn't merely a child making it all up. Then it was revealed that Grunkle Stan had known about the supernatural all along, and it all became clear to me. Grunkle Stan was merely pretending that he didn't know what Dipper and Mabel were talking about (you know, in order to protect them), but while he stood there putting on an air of scepticism for the kids, he was actually wondering what type of creature had made the footprint, and whether or not he needed to get his two kids out of there before it all went pear-shaped.
From the very beginning Alex has been hinting at Grunkle Stan's true persona, and it's simply amazing to see it all unfold and reveal it's true form.