Master Chief's the star of Halo. The game, that is. So he'd also be the star of Halo, the movie, should one ever get off the ground, right? Nope. Too hard! It's waaaay too hard to try and tell a story through a giant man in armour whose face you can't see so, as Bungie writer Joe Staten tells N'Gai Croal, they weren't even going to bother. Instead, Master Chief would have been relegated to the ranks of the supporting cast, while humans whose emotions you could see "carried" the film:
...over time I think everybody around the table agreed that the Master Chief is best left as the most important supporting cast member. Where the Master Chief doesn't have a face, but he has a whole body to emote with, whether it's his spine, or his shoulders. or the tilt of his head, or the way he slumps or reloads his weapon. There are these kinesthetic responses that he'll have which will really easily communicate the character and what he's feeling. That's hard to carry as a main character for an entire film. But you can certainly surround him with people who don't have helmets on and you can see their faces. They're normal actors doing their thing.
Hey, so long as one of those people is Ron Perlman, that's not the worst idea we've ever heard.
And The Winner For Best Supporting Actor Is...As Master Chief? [Level Up]