Well I say to hell with finding the actor, unless you use the guy who played chewbacca, or any other 7 foot tall actor. I say a fully CGI chief, voiced by Steve Downes, I think it would be the only way to put across the abilities of the Chief. I mean running in a ton of armour at 30km an hour incredibly agile and strong. I think am actor, no matter how much they are in shape or even if the armour was plastic, I think they couldn't do it.
If it has to be an actor I would say an unkown as I think most stars could portray the chief very well.
Harrison Ford? He's over 60 years old!!!
David Prowse? Also nearly 60!
Gerard Butler?(Guy from 300) Close, but John spends most of his time silent and not shouting. Though it would be funny if the chief shouted as he ran at the covenant 'THIS IS SPARTA!'
Jason Straham? He is an east-end geezer! Not a seven foot tall super solider.
Tommy Lee Jones as Keyes, hell yeah!
I wish Johnson could be played by the sargent in aliens he was based on.
But I would have to say either Samuel Jackson or Jamie Fox
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Halo: The Unlatched Casket- The horror is released........