On April 1st 2015, fror 1 hour only, Alex Hirsch was "possessed" by Bill Cipher on reddit, and hosted an AMA (ask me anything).
It ended up having just over 4000 comments, which we have picked the best and the most ‘friendly’ ones from to post below. 
Enjoy!
 

He opened with this, as to let redditors know it's him:

ALRIGHT KIDS TAPE YOUR EYEBALLS OPEN and light SOMETHING ON FIRE because for the first time in 1,000 years you're about to get a GLIMPSE BEHIND THE GREY CURTAIN OF REALITY into the KALEIDOSCOPIC ANARCHY THAT LIES BEYOND! HISTORY'S GREATEST SAGES would TRADE THEIR BLOOD AND LIVESTOCK for an opportunity to SEEK MY UNHOLY KNOWLEDGE but instead I'M GIVING IT TO YOU- THE EAGER, PASTY, ANTISOCIAL TROLLS OF THE INTERNET! WHY? FOR THE SAME REASON PROMETHEUS GAVE MAN FIRE- TO WATCH HIM BURN!

FOR the next 60 MINUTES you can GRASP the UNKNOWABLE with your CHUBBY LITTLE HUMAN FINGERS and ask me ANYTHING YOUR SICK MIND DESIRES! YES, even YOU Chris!

*ANYONE WHO PHOTOGRAPHS THEMSELVES WEARING A TIN FOIL HAT WILL BE ANSWERED FIRST!

*I WILL TAKE OFFERINGS IN THE FORM OF BENT FORKS, DEFACED DOLLAR BILLS, OR RECORDED SCREAMS!

Q - puma shirt or panther shirt?
A - THE ERADICATION OF ALL OXYGEN ON THE PLANET LEADING TO GLOBAL ASPHYXIATION!

 

Q - what do you know about a "magical" mailbox in the woods of Gravity Falls?
A - I KNOW THE GUY ON THE OTHER SIDE IS A REAL BLABBERMOUTH

Q - Can I hug you?
A -ONLY IF YOU CAN FIND ME! 25.0000° N, 71.0000° W

Q -WHY DO YOU TYPE IN ALL CAPS
A - I THINK IN ALL CAPS

Q - William, as you are a dream demon, can you tell me how to achieve my dreams? 
A - NO BUT I CAN INVADE THEM

Q -So. Bill. We all know pain is hilarious, but what is the MOST hilarious kind of pain?
A- YOURS!

Q -What's your opinion on the tapestry hanging in the Northwest's mansion? You know, the one with you and the bowing people and the fire.

A-IT CAME OUT PRETTY NICE, DONT YOU THINK?! THE NATIVE PEOPLE OF GRAVITY FALLS REALLY HAD A FLARE FOR CAPTURING MY TRUE ESSENCE. ALTHOUGH THEIR TIMING ON PROPHECIES LEFT SOMETHING TO BE DESIRED! GOTTA HAND IT TO THAT SHAMAN THOUGH, HE WAS A REAL FUN GUY TO TALK TO, EVEN IF HE TURNED OUT TO BE LESS COOPERATIVE THAN I HOPED. HEY, THERE'S ALWAYS THE NEXT GENERATION...

Q -Can you draw us a self-portrait?

A -I ALREADY DREW ONE IN YOUR GEOMETRY TEXT BOOK!

Q -Are you acquainted with Time Baby or know of his existence?

A -AUGH! DONT REMIND ME OF THAT OBESE BOMBASTIC DIAPER-WEARING TYRANT! THINKS HE'S SO GREAT JUST BECAUSE HE'S THE LAST SON OF AN EXTINCT RACE OF TIME GIANTS. WHATS THE POINT OF RULING THE ENTIRE FUTURE IF YOU DONT HAVE OBJECT PERMANENCE AND GET DISTRACTED BY JINGLING KEYS?!

I'D DISSEMBLE HIS MOLECULES TO SALT MY MARGARITA IF I COULD, BUT ITS NOT EASY. LET ME ASK YOU- HAVE YOU EVER TRIED TO PUNCH A 9 TRILLION TON TODDLER? YOUR FIST GETS STUCK IN HIS CHEEK FAT FOR LIKE A WEEK! IF I HAD PHYSICAL FORM....

Q- Dear Mr.Cipher, if you are an all-powerful being with no weaknesses, why did you need to make a deal with Dipper to possess his body? Doesn't that put a limit on you and prove that you're not all powerful?

A- ALRIGHT, YOU GOT ME, I MIGHT HAVE OVERSTUFFED MY RESUME! HEY, THEY SAY DRESS FOR THE JOB YOU WANT, NOT THE ONE YOU HAVE! BUT JUST BECAUSE MY POWER HAS A FEW BLIND SPOTS NOW DOESN'T MEAN I'M NOT ANGLING FOR A PROMOTION, YOU DIG? BETTER START STOCKPILING THE ESSENTIALS SMART GUY!

Q- What's your favorite place to people watch in Gravity Falls?

A- FROM WITHIN THE SKULLS OF THE GULLIBLE!

Q- How would a girl like me go about crushing her enemies?

A- TEAM UP WITH ME. INCREASING MY POWER IS EASY AND ANYONE CAN DO IT! JUST DRAW MY FORM ANYWHERE IN YOUR HUMAN WORLD- EACH IMAGE OF ME ACTS A PEEPHOLE FROM MY DIMENSION TO YOURS. THE MORE I SEE, THE MORE I KNOW. THE MORE I KNOW, THE GREATER MY POWER. THE GREATER MY POWER, THE MORE ENEMIES OF YOURS I CAN CRUSH. JUST SHAKE MY HAND

Although a lot of his answers are silly, many do hint some interesting things about the shows lore! If you want to see more of them, you can check them out here, but a word of warning: Some answers are NOT child friendly, and quite disturbing.