A golden harp can be seen twice throughout the movie--once in a flashback when a giant is stealing it, and again when Jack is in the room with all the treasure. In the Jack and the Beanstalk fairy tale, a harp that plays by itself is one of the items Jack steals from the giant.
Creating the giants took four main steps. The first step was using performance capture, which was used to capture the actor's facial/body movements and render them in a real-time virtual environment. The second step took place during principal photography, where Simulcam technology was used to help the human characters virtually interact with the giants that were rendered earlier. The third step was a second performance capture shoot to adjust giants' movements to seamlessly fit the live-action performances. The final step involved putting the finishing touches on the giant's animation, skin, hair and clothing, and composition in the shots.
Although not used in the theatrical release, Julie Andrews recorded "The Giants Fable" for the opening sequence for the film. This voice-over can be found in the deleted scenes on the Blu-Ray special features.
Screenwriter Darren Lemke first proposed the idea of updating the "Jack and the Beanstalk" fairytale in 2005. Lemke described the script as "a male-oriented story of a boy becoming a man" and drew a parallel between Jack and "Star Wars"'s Luke Skywalker.
Director Bryan Singer was interested in using motion-capture VFX for the giants, saying, "It takes you back to play-acting as a kid in your living room because you are running around, having to imagine that you are in Gantua, and there are all these giant things when there's nothing there. In that way, it's fascinating."
Creating the beanstalk involved two main requirements: set extension for shots of the actors interacting with the beanstalk (shot against a bluescreen), and complete CG renderings for shots of the beanstalk growing and extending from Earth into the world of the giants.
The trivia items below may give away important plot points.
At the start of the final battle, Giant King Fallon addresses his four generals by name: Fee, Fye, Fo, and Fum. These names together are the first line of the giant's singsong chant in the traditional "Jack and the Beanstalk" story.
Most tellings of "Jack and the Beanstalk" begin with Jack's mother sending him to sell the family cow. This version supplants mother with uncle and cow with horse, a change acknowledged in the humorous voice-over towards the end.