...I've never known how to play it. Had the module, but no manual. It was almost as bewildering as Raiders of the Lost Ark. Didn't help that I hadn't, and still haven't, seen either movie (honestly...). Sadly my favourite 2600 games are rarely mentioned any more (most are by Imagic and Activision, not Atari).
Dragon's Lair and Kaboom FTW!
The ET movie had essentially nothing to do with the game. Seeing or not seeing it would in no way change your understanding of the game.