We are all familiar with the couple's heart-breaking story, following husband Harry being tempted by an office-romance with secretary Mia and foolishly going to Selfridges to buy her a Christmas present.
Culminating in, what must be the most heart-wrenching scene in rom-com cinema, Karen finding out about her husband's antics.
Karen is given a Joni Mitchell CD for Christmas instead of the expensive heart-shaped necklace she found in her husband's coat and in true British stiff-upper-lip style, knowing it is under someone else's tree, she carries on as if nothing has happened.
Ever since the credits rolled over a decade ago, fans have wondered what really happened between Harry and Mia, and did Karen stick by her husband?
Last weekend, Emma Freud, who worked as a script editor on the 2003 film, and is real-life wife of director Richard Curtis, confirmed fans' worst suspicions in a live Q&A session.
One fan asked 'Did anyone ask if Harry ACTUALLY had an affair or was just tempted to? Alan Rickman villains are usually lovable but not this one!'
@carlotta429 DEFINITELY had an affair. i begged richard just to make it a flirtation, but no. the whole way.— emma freud (@emmafreud) December 13, 2015
Breaking hearts across the nation, Emma confirmed; 'DEFINITELY had an affair. I begged Richard just to make it a flirtation, but no. The whole way.'
Whilst Harry definitely did do the dirty on his wife, the pair stayed married and living together.
@JPerlstrom they stay together but home isn't as happy as it once was.— emma freud (@emmafreud) December 13, 2015
Emma went on to divulge many other previously unknown film secrets. In particular she discussed that many of the extras cast in the film were in fact members of her own family!
The housekeeper for Hugh Grant, who plays the Prime Minister, was her own mother, whilst the second lobster in the nativity was her daughter, Scarlett. Her son Jake also played the nativity king with the Spiderman face paint.
Sadly for fans of Emma Thompson's character, the whole affair could have been avoided if the original script had been approved, as Rowan Atkinson's bumbling sales assistant character was meant to take so long to wrap the gift that Harry was unable to buy it.
Emma also divulged that January Jones, who played the blonde American girl, wrote most of her own lines in the scene with Colin Frizzle in a Wisconsin bar.
Finally, those heart-warming scenes at Heathrow airport were in fact real families, not staged as one might expect from such a Hollywood blockbuster.
We know which film we'll have on the box this Christmas!