According to the book, Harry had green eyes. They initially gave Radcliff green contacts to wear, but he was allergic.
The fictional character was described with buck teeth. Emma Watson attempted to wear fake teeth, but had difficulty speaking with them while filming.
Upon casting, they recruited children from schools in Britain. It wasn't until Emma's teacher convinced her to audition for the role, that she got it.
Rowling requested the staff be entirely British
All the food in the Great Hall scenes was entirely real
14 Ford Anglius were needed to accomplish the scene of the boys crashing into the car
If you recognize the time Draco says "I didn't know you could read," to Goyle, the line was entirely improvised and hilarious
The role was originally offered to Hugh Grant
Michael Gambon went on to replace the original actor who passed away shortly after the 2nd installment. But many believe it was Ian McKellan who took the role due to this time as Gandelf
As professor Snape, he wore black eye contacts for filming