Here's how Michael Myers now-iconic head tilt in Halloween came to be. The original Halloween was a low-budget, independent production that would become a word of mouth horror smash. The movie was originally titled The Babysitter Murders until the producer hit upon the brainwave of setting it on Halloween night and calling it Halloween. Director John Carpenter was given free rein creatively - including writing and composing the score - and its success would lead to a slew of gory slasher movies in the 1980s, including Sleepaway Camp and The Burning.
John Carpenter's twist with Halloween's killer Michael Myers was to give the character a supernatural edge, where it's unknown if he's actually human or something else. He hired friend Nick Castle to play Michael for the majority of the character's appearances, mostly due to his background in dance. Castle's eerie movements and body language would come to define the character. He wasn't asked back for the majority of the sequels, where a succession of stuntman would play Michael, although he made a cameo in the role for Halloween 2018. He's also returning for upcoming sequel Halloween Kills.
One reason John Carpenter selected a blank, featureless mask for Michael Myers in Halloween is to suggest he needs to wear it just to pass for human. Nick Castle was given little in the way of direction for Michael, which often just consisted of being told to stand still or walk from one point to the other. One of the few pieces of direction came in the scene where Michael has murdered Lynda's boyfriend Bob. Michael has pinned him to the wall with a butcher knife, which is when Michael starts tilting his head slowly from left to right, seemingly admiring his work.
Nick Castle would later explain the origin of Michael's head tilt in an EW interview, where Carpenter instructed the actor to turn his head during the scene. Castle also explained he didn't understand the significance of the moment until he saw Halloween. The move also suggests something childlike within Michael, which makes a certain amount of sense since he spent years in an asylum from the age of 6.
The famous head tilt would reappear in later sequels, and when Nick Castle returned for his Halloween 2018 cameo, his scene consisted of Michael and Laurie Strode (Jamie Lee Curtis) seeing each other again for the first time in 40 years; this leads Michael to cock his head in recognition. For such a simple gesture, Michael Myers' head tilt carries a lot of power in Halloween, which is why it became such an iconic moment.