Here's all the inside scoop:
1. The concept came about when Jessica's uncle Julian read a book called To Marry an English Lord by co-authors Gail McColl and Carol McD. Wallace. In the post-Civil War until the first World War, American heiresses went to England to swap titles for cash.
2. Julian writes the entire script for each episode. In the beginning, they had a crew of writers, as is usual for such shows, but that didn't work.
3. Jessica Brown Findlay, who played Sybil, came to Julian and said she wanted to leave the show. "Do you mean 'leave and come back' or 'leave permanently'", he asked. She said, "Leave permanently." And he said, "Sounds like death in childbirth to me!"
4. Many of the characters are patterned after Fellowes family members/friends of family/or tales told to Julian. For example, a friend came over to Julian's house in London and said, "I've been cleaning out old papers and I just came upon the most extraordinary letters. Seems my aunt was at a house party and a man died in someone's bed. Well, the only way to keep it secret was for all the women--and they were on one floor as custom demands--to help haul his body out of that one woman's bedroom and back down the stairs up to his own bed! Most amazingly, the next day the men were talking about how weak women are!" As Jessica said, "A problem shared is a problem dumped on other people."
5. The three Crawley sisters represent the three types of women of that era: Mary wants power and will get it the old fashioned way by marrying it; Sybil wants power and was willing to be involved in the political system; and Edith wants anything she can get! (Poor Edith.)
6. Mary is patterned after Julian's mother, and Cora's remarks to her daughter on her wedding night is the same as Julian's grandmother's to his own mother: (Paraphrasing here) "You realize there are things an English wife must do. But no one tells you...it's the most horrific fun!"
8. The producers and Julian have purposefully held back from putting any realistic new products in the house and the scenes because even if they were appropriate for the times, because they are new, they are jarring to the viewers.
9. In the UK, Downton Abbey is interrupted by commercials. Julian much prefers that it be viewed the way we see it here in the US.
10. O'Brian will not be coming back. She's a copy of a real ladies maid who ran off an entire family, one by one, so that her mistress died alone and thinking she'd been deserted by her loved ones.
11. Julian's wife helps him with ideas. She's the one who suggested that Bates have a limp and be injured. (Wasn't that brilliant?) Just love the relationship between him and Anna!