[OOC] 
Backtagging: Always!
Threadhopping: Only if the current thread is too cluttered.
Fourthwalling: No.
Offensive subjects (elaborate): N/A

[IC]
Hugging this character: You can try.
Kissing this character: Well he doesn't have lips... so...
Flirting with this character: Go for it.
Fighting with this character: Yes!
Injuring this character (include limits and severity): Yes. Anything goes.
Killing this character: Sure -- just talk it out with me first.
Using telepathy/mind reading abilities on this character: Go for it. Just be careful, he thinks backwards.

THE IMPORTANT STUFF: Bill Cipher claims to know everything. Of course, that only extends to the people of Gravity Falls. However, when it comes to cross-canon excursions, his knowledge drops from a rough 85 to about 0.

So! Regarding FOURTH-WALLING.

1) Is Bill Cipher allowed to visit your character via nightmares?
2) Is he allowed to gain information about your character through these visits? (Personal history, possible abilities, etc.)
3) If "yes", how much is he allowed to see? If "no", ignore this question.

ONE MORE THING!

Bill also has the ability to possess people's bodies if they make a deal with him and shake his hand. So of course, he needs consent. If you'd like to play something of this nature out, please PM me!

Something else come up in RP? Ask away!