Kit Harington is a direct descendant of Robert Catesby on his mother's side. Harington's full birth name is Christopher Catesby Harington.
In real life, Lord Robert Cecil was hunchbacked, and modern day physicians have diagnosed him having had scoliosis. This explains his irregular posture in the show. Cecil was 5'4" tall, but is played by Mark Gatiss, who is 6'1" tall.
Sir Willian Stanley (1548-1630) was denounced by Fawkes but pardoned by Sir William Cecil. He was a descendant of Sir William Stanley, whose late intervention in the Battle of Bosworth Field on August 22, 1485 won the crown for Henry Tudor (who became King Henry VII).
Kit Harrington is related to King James I, the target of the assassination attempt, and to Lord Harington of Exton, who was in the Houses of Parliament that Catesby and his co-conspirators tried to blow up, on his father's side. He is related to Catesby on his mother's side.
Part of the series was filmed at Oakwell Hall, located in Birstall, West Yorkshire.