During casting calls, director Jim Sheridan recalls that after a long day of auditions, he was very glad to have found Emma Bolger to play Ariel. He said that that little girl "told" him that he had to cast her sister Sarah Bolger as Christy. Sarah was only 10, and he had wanted Chirsty to be 12 or 13. After exhausting all other options, he auditioned Sarah for the part of Christy and she turned out to be so impressive that he didn't mind about her age.
The child actors (Sarah Bolger and Emma Bolger) called "cut" and "action" in every scene. This was decided by the director and the girls early during filming, so that it would make the acting easier for them.
Originally written to take place in 1982 when E.T. the Extra-Terrestrial (1982) first came out, budget cuts obligated it to be set in 2002 when it was made. Coincidentally, the 20th anniversary edition of ET played in many cinemas in 2002, so that plot point didn't have to be changed.