Wood had worked with a teacher three weeks prior to going to Barcelona and found it stressful having to play the piano and speak at the same time saying, "It was incredibly technical [...] lots of moments where it was jumping from where I'd play, listen to a click, listen to music, have to be in the right place and the right time and hear dialogue and repeat dialogue".
The trivia items below may give away important plot points.
Most of John Cusack's dialogue occurs while his character is off-screen, speaking to Elijah Wood through an earpiece. When Cusack finally appears on-screen, towards the film's end, he speaks just one line of dialogue before he and Wood engage in a non-speaking hand-to-hand battle.
By the beginning of the movie, Tom Selznick (Elijah Wood) is afraid a great catastrophe might fall upon him, despite everyone he speaks to wishing him to "break a leg". The expression is used in theatre to wish a performer "good luck" in an ironic way. And after the catastrophic events of the movie, from which Tom rises with tremendous success, he actually breaks a leg, making the irony of the theatrical wish true. The leg in the phrase break a leg refers to one of the curtain supports. It means you get called out for so many bows due to the audience cheering that the leg breaks.