I think there’s a good mix of both that make the aliens seem more real. It should be a live action muppet, with CG facial movements, to really get a realistic feel, because you’ll have something to work with, not just a stand-in or green stick. You can actually see what you’re looking at and where to stare when acting with it. If it’s a background character, you can either do CG or a live action muppet. I prefer the muppet, because you get a realistic shadow cast from the muppet or an actor in a suit. That’s what they did for Simon Pegg in TFA.