Doesn't matter if you're a good actor, if the material sucks it's irrelevant. This isn't some deep, thought provoking drama. It's a F'n monster movie. Trust me, Brody.. hell NO ONE in this film is getting nominated for a Best Actor/Supporting Actor award for their performance in this movie. Unless it's an MTV Movie Award.
How good was Ben Kingsley (another Oscar Winner) in Species? Oh yeah.. he sucked. Because the script was garbage.
You act like just because someone is considered a 'Good Actor' they can magically make any script gold. Doesn't work like that. It's all about the material they have to work with.. and again, this is a monster movie. So it's not about the acting.. it's about the monster i.e. The Predator.
No one is going to this movie to think or see some 'really great acting'. Seriously. They're going to see some Predators kill people. Really basic ish. That's the appeal. It's a popcorn movie, not a multi-layered, twist filled head scratcher.
First off, guarantee you won't find many Marines/Soldiers in general that look like Arnold Schwarzenegger in real life. And what exactly do you think a mercenary is!? A lot of them are ex-military. So I'm not sure why you're acting like a Marine = "some big huge man" and a Mercenary = a skinny, goofy looking twat that always looks like he's about one insult away from crying ala Adrien Brody. The Marines in Aliens weren't muscle bound freaks. Most military movies don't have anyone that looks like Schwarzenegger. Who said the lead had to be an Arnold clone anyways!? I think it's funny you keep focusing on that. Hell Walton Goggins isn't much bigger than Brody and even HE would've been more believable in that role.
What's next? Kate Winslet as the lead in a new Aliens movie? Awesome.