Katherine Heigl's hair is a wig. She wanted it to stay true to the book's description of Stephanie, and it would've been difficult to get her real hair to look like that.
The film was a box office disappointment for all involved. Plans for a big screen adaptation of "Two for the Dough", the next book in the series, were scrapped.
The film is based on Janet Evanovich's 1994 book "One for the Money." Evanovich has published 25 novels in the Stephanie Plum series. The 25th novel, "Look Alive, Twenty-Five", was published in 2018.
Janet Evanovich told USA Today that "Katherine Heigl is the perfect Stephanie Plum," and that whenever she writes about Stephanie Plum, she sees Heigl's face in her head.