In this video, E.T. Casting Director Marci Liroff explains how she cast Henry Thomas in the lead role. It's a surprisingly moving story, involving Dungeons & Dragons, very emotional improv, the working title of the film (keep an eye out at 0:35), and more. This is short, and really cool:
And now, three minutes of raw footage from Thomas's audition, showing how the improv scene we saw above developed. Truly amazing.