Takeaways from Reddit user InfernoBA:
The phrase "summer snow" was stuck in his head when he first began writing AGOT and he came up with the seasons being unsteady from that
He initially made a list of the names and dates of previous Targaryen kings, and began fleshing them out in his head as he did on paper
He was in the middle of writing ACOK when he wrote The Hedge Knight. He attributes the significant boost in sales from AGOT to ACOK to THK gaining him attention in the anthology it was published in.
He's never destroyed/deleted any chapter that he decided to scrap or rewrite.
He might release Tyrion's scrapped chapter(s) where he encounters the Shrouded Lord in ADWD as a standalone novella someday after he finishes the main books
TWOW --> DE 4 --> ADOS --> DE 5 with F&B 2 at "some point in there" is the plan
He contractually had the option to write an episode of GoT every season. In Season 5 and onward he said the story was too different and writing entirely new scripts instead of adapting book scenes would take too long ("six weeks" minimum), so he stopped writing episodes. This one's interesting because I wonder what an episode of Season 8 written by him would be like.
Some things in the show's ending are exactly like the books and some others will be different
GRRM initially suggested the idea of taking a break from TWOW to do F&B, and his publishers agreed
Here are the books by Thomas Costain that GRRM said inspired Fire and Blood. History but with narrative and intrigue. All four books in the set are on sale for 99 cents each on kindle. https://www.amazon.com/Thomas-B-Costain/e/B000ARBJ1C?ref=sr_ntt_srch_lnk_1&qid=1569955293&sr=8-1