The novel is set in London. The movie changed it to New York City to connect with an American audience. Following the success of Sex and the City (1998), New York City was seen as a selling point to female audiences.
Isla Fisher researched her role by visiting some overspend/under earner groups, people whose lives have essentially become unmanageable because of their compulsive shopping.
Reese Witherspoon, Jessica Alba, Jessica Biel, Anne Hathaway, Amy Adams, Emily Blunt, Kirsten Dunst, Katie Holmes, Amanda Bynes, Rachel McAdams, Amanda Seyfried, and Lindsay Lohan were all considered for the role of Rebecca Bloomwood. Witherspoon turned it down because she thought it was too similar to her Legally Blonde (2001) role.
Dame Kristin Scott Thomas revealed in 2014 that she didn't like the experience of filming. "I thought it would be quite good fun. But I spent my entire time waiting. I hated it, hated it, hated it, and I said that I wouldn't do another one. Funnily enough, I haven't been asked to."
The Finnish man that meets Rebecca at the Cocktail Party doesn't have his Finnish subtitled, but what he was saying was: "Hi! So nice to meet another Finn in here! Ever since I've been here in Americ-" before Rebecca cuts him off by slapping him.