The technology to create an immersive sandbox-style game experience was a rare feat in the days of the PlayStation 2. However, in this generation of super-power consoles like the Xbox 360 and PlayStation 3, it is much easier for developers to wrap their heads around creating an entire game world, rather than just a series of events.
It was this technology that helped developer Rockstar Games create the stellar Grand Theft Auto 4, and it is that very same game engine that will power Red Dead Redemption. This means that players will be able to interact with the world in a similar fashion as they would in real life (if they lived in 1908, that is). Choosing missions at will, being a good guy or a bad guy, and even general exploration will be a big part of the experience.
There will be an advanced in-game monetary system, so buying things like horses and weapons will require some sweat on the part of your character. Also, like GTA IV before it, Redemption will employ a cover system, allowing players to duck from gunshots behind whatever happens to be populating their immediate vicinity.