British Accents: In what may be the finest British Accents twist of all time, author Bernard Cornwell revised the backstory of the character Sharpe to reflect Sean Bean's portrayal. The books had established that Sharpe was from London, but Bean is from Sheffield and has a distinct Northern accent; Cornwell established in later novels that while Sharpe had indeed been born in London, he fled to Sheffield for a life of crime to avoid being sold as a chimney sweep by the person that ran his orphanage.
The Other Darrin: Wellesley is played by David Troughton. His elevation to the peerage not only changes his name to Wellington, but his actor to Hugh Fraser.
Mr Sweeny shows up in Sharpe's Waterloo as one of the Duke of Wellington's relations.
Hey, lots of guys and one gal later appeared on Hornblower.
Josefina, a Spanish aristocrat, is Senora Ortega (Katia Caballero).
Major Nairn is dying Captain Keene (Michael Byrne).
Colonel Fletcher is Lieutenant Buckland (Nicholas Jones)
Matt Truman is Gunner Hobbs (Philip Glenister).
Mr Tapling of the diplomatic service is Runciman (Ian McNeice).
Napoleon, Boney himself, is Steward James Doughty (Ron Cook). Holly crap, no wonder sailor Styles hated him so much.
What Could Have Been: PaulMcGann was originally cast as Richard Sharpe, but he broke his leg while playing football only a few days into the shooting of Sharpe's Rifles. Sean Bean was recast in the role and... well, you know the rest.
Writer Revolt: An editor told Bernard Cornwell to change a scene where an Ensign died likely because ensigns are frequently young boys or men in their mid to late teens. He resented being told how to write, so he changed it to be more depressing. And in a number of the books since, Cornwell has killed off Ensigns in increasingly worse ways.
Writing by the Seat of Your Pants: Cornwell does plan his novels in advance, but has admitted to changing them up midway through because he thinks it reads better. An example - Frederickson was supposed to marry Lucille in Sharpe's Revenge.