For starters, Axis Powers Hetalia originated as a web-manga by a guy named Hidekaz Himaruya. The basic premise of the comics was that it follows the anthropomorphic personifications of certain countries (particularly those involved in WWII, hence the "Axis Powers") as they go about their lives. It's largely a satirical piece, using comedy to address well-known and lesser-known historical facts alike. Historical political and military interaction between countries is represented in Hetalia as social and romantic interaction between the characters; the webcomic version in particular depicts military and economic incursions using innuendo. The "Nations" themselves are based off of several of that country's stereotypes, played up for laughs of course (such as Germany being a very strict, by-the-book kind of guy, Japan always being quiet and polite, France is a sex addicted pervert, etc).
Now I'm fairly certain a live action Hetalia movie/TV series will never come to fruitation. This is mostly a response to these other casting calls from the video site NicoNico, which were all fun and interesting to watch, but certainly not the best thought out of lists. So I decided to make my own visualization of what the nation-characters would look like in real life.
I added annotations to certain casting choices describing why I chose an actor/actress for the part I gave to him/her. Pause in between to read them, or you'll miss out. Enjoy!