James Franco said that Ashton Kutcher, Mila Kunis' husband (fiancé then), couldn't endure watching the scene where Franco had to slap Kunis' face and drag her across the floor. Kutcher had to leave the room during the shooting. "I mean, it wasn't my idea!! It was the script!" Franco said. Further to the close of this scene, when dragging Kunis out across the rug, Franco's stumble at the end was unscripted: but director Haggis felt it suited the scene's intensity so left it in the final take. [Latter direction reference from director's own DVD commentary]
The scene where Anna shows up at Michael's hotel room and gives him her robe took about 57 takes because Liam Neeson kept getting distracted with Olivia Wilde's naked body.
There are several misplaced "items" in the movie. Olivia Wilde's character is supposed to be in Paris, but she can briefly be seen in the back seat of a car driving by the Bar Americano. One of the paintings in James Franco's studio is on display in Paris. The obvious one where the note Mila Kunis writes in the New York City hotel, is picked up by Wilde in Paris.
Adrien Brody's character, Scott, is always saying "spasibo," which means "thank you" in Russian, although he is in Italy.
Mila Kunis was originally offered the part of Anna, but when she read the script, she asked to play Julia instead.