“Canon” refers to what is explicitly stated in the books.

Such as: the fact that Lestat killed 8 wolves (”And the pack was eight wolves, not five as the villagers had told me.” (TVL)) on that fateful day, earning the “Wolfkiller” nickname.
“Headcanon” is what books/information you personally believe in, that in your mind have occurred in the series. For more on that, hit the jump.
This is partly bc some people most of us think that the VC went off the rails at a certain point, and they didn’t read, or they read and disagreed with, the events that occurred in some of the later books.
So, for example, someone who stops at Memnoch the Devil (#5) will not accept certain events or ships that occur after that book in the series.
This is where we employ #Your headcanon may vary, and #Ship and let ship.
- If person A accepts all of the books in the series as their “headcanon,” that means that they believe in all of the events and ships that have been written.
- If person B only believes in books 1-4 as their “headcanon,” that means that they believe in only the events and ships that occur up until that point. (People can skip around, they might accept books 1-3, 5, 6, and 8. Or maybe just 1-4. It all depends on your own imagination!)
If A and B want to RP as the same canon character X, that character will be different based on what X has experienced in canon. Person A’s portrayal may include information and characterization that Person B’s will not, and vice-versa.
That’s partly why people ask about your “headcanon,” to find out whether your muse would be compatible with theirs. After all, if they are RPing as character Y, who first appears in a book you do not accept as canon, then you might have to agree to compromise with them, and find a way to make it work, or you might not be able to RP your muses at all.
We also have a situation here where there are discrepancies between books, we have unreliable narrators who can’t be 100% trusted… such as the scene near the end of IWTV when Louis follows a young vampire to Lestat’s run-down house in NOLA, in which Lestat is crying like a little old lady:
“I could see the tears welling in his eyes; and only when his mouth was stretched in a strange smile of desperate happiness that was near to pain did I see the faint traces of the old scars. How baffling and awful it was, this smoothfaced, shimmering immortal man bent and rattled and whining like a crone.” – Louis, IWTV
Lestat calls bullshit on that moment:
“ ‘Ah, that makes you out to be a perfect liar,’ I said furiously. ‘You described my weeping in your miserable memoir in a scene which we both know did not take place!’ “ – Lestat, TOBT.