Here are a few very interesting Rowling quotes for your reading pleasure. I’ve added my own thoughts here and there.
About the first chapter of Philosopher’s Stone:
“The very, very earliest drafts of the first chapter of ‘Philosopher’s Stone’ have the Potters living on a remote island, Hermione’s family living on the mainland, her father spotting something that resembles an explosion out at sea and sailing out in a storm to find their bodies in the ruins of their house.” (www.jkrowling.com)
Interesting that she wrote Hermione’s family much more fully into the story at first. I wonder what names she gave Hermione’s parents, since by the final version they had none? Was that when Hermione’s last name was still Puckle?
About the future relationship of Dudley and Harry:
“… after Dudley’s brave attempt at reconciliation at the start of Deathly Hallows, the two cousins would have remained on ‘Christmas Card’ terms for the rest of their lives, and that Harry would have taken his family to visit Dudley’s when they were in the neighbourhood (occasions dreaded by James, Albus, and Lily).” (www.jkrowling.com)
Oh, how I want to see this scene. Tell me someone has written a fanfic of it …
About Hagrid’s accent making him sound stupid:
“Hagrid’s sort of West-country-yokel, which is the part of Britain where I grew up, I didn’t grow up in Scotland, I grew up on the border with Wales. So Hagrid’s [is] kind of the accent English people always put on to sound stupid. Hagrid isn’t stupid, but he’s got that kind of very country way of speaking.” (WBUR Radio interview from 1999)
Full discloser: I did some editing of this quote to make it more readable. It was a literal transcription of spoken word, with all the ums and repeats included. I was very careful to maintain the meaning.
About Harry’s erstwhile extended family:
“As a writer, it was more interesting, plot-wise, if Harry was completely alone. So I rather ruthlessly disposed of his entire family apart from Aunt Petunia. I mean, James and Lily are massively important to the plot, of course, but the grandparents? No. And, because I do like my backstory: Petunia and Lily’s parents, normal Muggle death. James’s parents were elderly, were getting on a little when he was born, which explains the only child, very pampered, had-him-late-in-life-so-he’s-an-extra-treasure, as often happens, I think. They were old in wizarding terms, and they died. They succumbed to a wizarding illness. That’s as far as it goes. There’s nothing serious or sinister about those deaths. I just needed them out of the way so I killed them.” (2005 TLC interview)
There is conflicting canon information about the age of James’s parents, actually. According to the Black Family Tree, Dorea Black, who appears to be James’s mother, was only 57 when she died in 1977. Perhaps his father, Charlus Potter, was considerably older — his birth and death years are not given. It’s a bit startling to realize that every single person that Harry sees in the Mirror of Erised was dead by the time Harry was eleven.
You thought he saw only his mom and dad? Nope, that’s the movie version confusing you. 
This is from PS12:
He had to clap his hands to his mouth to stop himself from screaming. He whirled around. His heart was pounding far more furiously than when the book had screamed — for he had seen not only himself in the mirror, but a whole crowd of people standing right behind him … There he was, reflected in it, white and scared-looking, and there, reflected behind him, were at least ten others. Harry looked over his shoulder — but still, no one was there. Or were they all invisible, too? Was he in fact in a room full of invisible people and this mirror’s trick was that it reflected them, invisible or not?
A few sentences later, this group was referred to as “the Potters,” but some were undoubtedly Evanses, since he saw “green eyes just like his” which he would have inherited from Lily’s side.
About the books “getting darker” as the series went along:
“I’m surprised when sometimes people say to me, ‘Oh, you know, the books are getting so dark.’ I’m thinking, ‘Well, which part of ‘Philosopher’s Stone’ did you think was light and fluffy?’ You know, there is an innocence about it, Harry is very young when he goes to the school, but the book opens with a double murder. The possibility of death, I think, is present throughout ‘Philosopher’s Stone,’ and I feel that there are a couple of really gruesome images in ‘Philosopher’s Stone.’ I think the first book contains more gruesome imagery than the second, despite the giant snake, because the cloaked figure drinking the unicorn blood is pretty damn creepy. It was to me when I thought of it, and I really, right up until now, all these years later, think that the idea of the face in the back of the head [Voldemort sharing Quirrell’s body] is one of the most disturbing images in the whole book. (The whole book; I call it one big book. In the whole series.)” (2005 TLC interview)
I remember facing this criticism of the books when Goblet of Fire came out. There was a group of parents who wanted me to remove the book from my library because of the graveyard scene. One of their arguments was exactly that: the books have become so dark. I don’t think that my pointing out that the other three were just as dark would have helped my cause.
About Fred, George, and the Map:
Q: How did [Fred and George] figure out how to work the map?
JKR: Don’t you think it would be quite a Fred and George-ish thing to say in jest, and then see this thing transform? Can’t you just see them?
Q: But the exact word combination? Is that just a lot of luck, or Felix Felicis —
JKR: Or, the map helped.
Q: Yep, yeah. You can see them sort of answering and joking with each other —
JKR: And the map flickering into life here and there when they got closer and closer, and finally they hit upon the exact right word combination and it just erupts. (2005 TLC interview)
This is one of those quotes that I had forgotten about. Glad I found it again! I’d always figured that the map would have sensed kindred spirits and somehow helped Fred and George work out how to do it. This really, really makes sense to me.
About something we never got to see:
And one NON-fact, to finish things off, from an interview in 1999, between books three and four:
“Yes, you will hear from Mr. Weasley’s car again, but … I’m not telling you how.” (Barnes & Noble chat, 1999)
No, we never did, sadly. I would have loved to have seen it careening out of the Forest, attacking Death Eaters in the Battle of Hogwarts. I wonder if that was what she had in mind?