you were a tourist at Gravity Falls you, of course came to see two of the main attractions, the Bill Statue and The Mystery Shack. While walking you find the a stone hand sticking out of a bush, you shake the hand, and everything turns bright, like you were Blinded, when it cleared, you saw Bill.. in his physical form. He doesn't notice you.

Bill: "wait.. I'm back..? I'M ALIVE! HOLY HECK I ACTUALLY SURVIVED THAT PUNCH!!! HAHAHAHHAHHAHAHHAHHAHAH!!!"