"That scene where my character is standing by the bed crying is so well known because it's something everyone's been through," she told The Telegraph.
Thompson was referring to infidelity, which she experienced with ex-husband Kenneth Branagh, who cheated on her with Helena Bonham Carter in 1994.
In the movie, Emma Thompson's character, Karen, discovers her husband (the late Alan Rickman) was cheating on her. After opening her own Christmas present, she realizes that a necklace she found in his coat is meant for another woman. So, she leaves the room and quietly sobs while listening to Joni Mitchell.
"I had my heart very badly broken by Ken," Thompson said. "So I knew what it was like to find the necklace that wasn't meant for me. Well, it wasn't exactly that, but we've all been through it."
Thompson and Branagh first met while making the 1987 BBC drama "Fortunes of War" and got married two years later. They starred together movies including "Dead Again" and "Much Ado About Nothing." Then, Branagh began an affair with Bonham Carter on the set of "Mary Shelley's Frankenstein." They were together for five years.
Now, Thompson has been married to Greg Wise, who she met on "Sense and Sensibility," for 15 years and has two children. Branagh married art director Lindsay Brunnock in 2003 as well. Bonham Carter had a longterm relationship with director Tim Burton, with whom she has two children.
"That is... all blood under the bridge," Thompson said of holding a grudge against "the other woman."
"You can't hold on to anything like that. It's pointless. I haven't got the energy for it ... Helena and I made our peace years and years ago ... She's a wonderful woman."
Nine intertwined stories examine the complexities of the one emotion that connects us all: love. Among the characters explored are David (Hugh Grant), the handsome newly elected British prime minister who falls for a young junior staffer (Martine McCutcheon), Sarah (Laura Linney), a graphic designer whose devotion to her mentally ill brother complicates her love life, and Harry (Alan Rickman), a married man tempted by his attractive new secretary. Read More