As a great first example is I had to really search my archives to find a photo of Zach not smiling. He was always smiling. Even when he was making my life miserable, like his brother does now, he had the best disposition. On the other hand you have James. He is a super happy guy, but as we all know, is not impressed with much. OK with anything. I have to search for a photo of him where he is smiling. He does not give it up as easily as his brother.
Another big difference is the running factor. James loves to run. He runs over walking any day and kicks balls over not kicking anything. Every morning he wants to play "football" which means we run through the house while he holds a football and Alex, Zach or I chase him and pretend to "tackle down". Zach is not and has never been a runner. I loved that about him as a toddler and especially now that I have a runner. I never worry about him running off in the street or shopping malls. He always preferred to be in the stroller than walking when he was little, and James is that way a bit as well. I am certain James' affinity for running comes from wanting to keep up with his brother, but it is so, so different than I am used to.
But man do they look alike.