Gravity Falls' Theories

Bill & Stan

Fans have been pointing out the similarities between Stan Pines and Bill Cipher since the triangle trickster was introduced. Their characteristics certainly mirror each other: they have similar ways of speaking, they're both wheeler dealers and showmen, and they're both snappy dressers. Of course, they're both voiced by Alex Hirsch himself. So why bother to make two characters so similar, especially if one is good and one is evil? This is the question fans have been trying to answer from the beginning, and there are plenty of theories to try and explain this.

The most interesting thing about the connection between Stan and Bill is the fact that despite everything Stan still doesn't know Bill Cipher exists. This is crucial: before S02E17 aired I pointed out the fatal flaw in Dipper not letting Mabel in on what he and Ford knew about Bill's plans. Obviously, this lead to her being easily manipulated by Bill (if she had known about the importance of the Rift, she never would have given it to Bill/Blendin). Clearly, not letting everyone in the Pines family know about Bill's plans just makes Stan and Mabel more vulnerable.

So what hints have we had so far that Stan and Bill are connected? The most obvious hint is how Stan and Bill often repeat each other's lines...

"Eenie, meenie, miny... YOU!" - Stan, in Tourist Trapped, choosing Dipper to put up signs. Also Bill, in Sock Opera, choosing Dipper as his literal puppet.

"Sixer!" - Stan's fond nickname for Ford in A Tale Of Two Stans. Also, Bill's nickname for Ford in The Last Mabelcorn.

"Growing up is optional." - Bill
"Just because you're growing up doesn't mean you have to actually grow up, y'know?" - Stan
There are a bunch of other little parallels, like the fact that Bill exactly mimics Stan's movements in the S02E18 alternate opening.

Suffice to say, the characters are very similar. So what does Alex Hirsch have to say about all of this? It seems he also wants to establish a link between them: he recently said this at New York Comic Con, as reported by several fans...

This also drops some very interesting hints about Bill's estranged family (in a Reddit AMA Bill replied "NOT ANYMORE" to questions about whether he has family... did he murder them?), as well as his ominous friends.

Alex Hirsch seems to be establishing yet another parallel between Bill and Stan... but what could this mean for S02E18?!

Strange Times Ahead
So far we definitely have an established connection between Bill and Stan, but very little information about why this is. It could of course be a red herring, or just an interesting narrative choice. But knowing Alex Hirsch's tendency to foreshadow all of his major plot twists, it's definitely possible that this connection will turn out to be crucial to the story. There are several possible reasons for the connection...

Bill needed Ford to trust him, and so used his memories of Stan to mimic Ford's brother - this is pretty obvious with Bill's use of Stan's nickname for Ford, "Sixer".
In fact, Bill could have used Stan to sabotage Ford's science project, ensuring he would end up in Gravity Falls.
Bill could be mimicking Stan in order to possess him (but seeing as he already got what he wanted by possessing Blendin, this theory may have been debunked).
Bill could actually be Stan Pines in some weird dimension bending, time travel twist.
Stan Pines could be Bill?!
Stan Pines could be Bill?!
Hey, anything's possible in Gravity Falls. Personally, I don't like the Bill-is-Stan theory because it would pretty much destroy Stan as a character, and it doesn't seem like something Alex Hirsch would go with. Bill mimicking Stan to gain Ford's trust is more likely, as the theme of trust has been very strong in season 2 (especially with Mabel in Not What He Seems and Dipper & Mabel vs The Future). And again, this could just be a narrative choice on Hirsch's part, but his NYCC comments seem to tease the future in the episodes.