We built a little home made vacuum forming table like this one on instructables, and used it to mold thin polyethylene sheets over clay. The polyethylene was white so we spray painted it bronze, then hit it with a wash of black paint to make it look worn. The rest of the costumes is just a big strip of red fabric (cheap as possible because you need at least 6 feet to get a nice billowy cape), and bits of fake leather.
Oh yeah, the last part of the build process involves thousands of push-ups and sit-ups so that you look like the guys in the movie, but we were kind of tired from vacuum forming so we decided to skip that step and have a beer :D
The other costume is Max from Where the Wild things Are. This was actually easier than I thought...
To make the pattern I just traced myself on a big sheet of paper, then added a bit of extra room to make it baggier. If you don't have a giant piece of paper you can always take a cheap t-shirt and pair of pants that fit you and lay them flat for the pattern. I got a rough idea for the hood by tracing out a hoodie. Same idea with the gloves, just trace your hands, cut out two pieces that size, sew the edges together, then flip the whole thing inside out to hide the seams.
I've found getting the pattern wrong is pretty easy, so I made a prototype out of the cheapest fabric that I could find at the fabric store. If it was off in any spots, I'd pin it in to make it tighter, or mark it up with a marker (e.g. "more room around the waist!").
Once I'd worked out the kinks, I cut the whole thing out of a big piece of whit furry fabric and made the final version. I cut the tail out of a furry bath mat that I found at Walmart and stuffed it with pillow stuffing. I'm really sloppy with my sewing, which is why I like Halloween costumes. No one is looking at the seams anyway! This costume was even more forgiving because the seams are hidden in white fluff.
In the movie, Max's wolf costume is filthy, so I ran the finished costume through the wash with some really dilute black clothing dye. This made it more of a grey than a pure white. Bonus: by the end of Halloween it actually was filthy, and it looked (and smelled) even more authentic!