Originally, Shepard wrote the character of Sky as part Native American, but because of Wenders desire to cast Sarah Polley, that aspect of the character was set aside. Both agreed that her being Native American was not essential to the character, and Wenders had wanted to work with Polley because he'd been so impressed with her acting in past projects.
The trivia item below may give away important plot points.
The picture of Sky's mother is a photograph of Sarah Polley's own mother, the late actress Diane Polley. Diane, who died in 1990, was a huge fan of 'Sam Shepard (I)'. Shepard plays the father of Sarah's character in the film.