One American I've met was a bit stereotypical in some regards. He was on a biking tour from Sweden to Palestine, had an unusual beard, huge white teeth, was extremely friendly and a bit loud, and he literally carried a bucket of peanut butter with him because he said that was the most efficient way to carry energy for his travel.
I was an intern at a software company that just got bought by a huge American company (Warner I think) and one time some executives were visiting, walked through our office complimenting all the developers loudly and then disappeared again.
My general impression of Americans I've met in person is that it's difficult to see what you guys really think and feel, because you seem to hide it behind a layer of aggressive cheerfulness. So when I see someone radiating that, I expect them to be from the US.