Most games generally fall into two camps. One camp is filled with choices, and its worlds will change based on your decisions; the other camp has no choices, and its stories will end the same way no matter who plays them. 

Though better examples may exist, I am going to focus on Fallout 3 to represent the choice side and Modern Warfare 2 to represent the “on rails” experience. 

The type of game with very few choices tends to be shorter and is mainly represented by shooters and action games. 

Various obstacles populate these games, and they usually just have one way past them. 

In Modern Warfare 2, that means killing bad guys, and sometimes you have a time limit. 

Though the game gives you many ways to kill the bad guys or sometimes lets you skip some, you never have the option to take prisoners, surrender, wound them, or take a different route to your objective (except for carefully plotted alternate paths, which are pretty clearly the way you were meant to go in the first place). 

In a way, this type of game has not evolved much since Mario, where the objective was to get to the right side of the screen, and you can jump on some enemies on the way if you feel like it. 

In fact, Mario games have long had more choice than games like Modern Warfare 2, letting you bring a variety of power-ups into almost any level and use warp whistles and secret pipes to skip around to different parts of the game. 

Modern Warfare 2 just consists of a sequence of very carefully scripted events where you use whichever weapons they want you to use on the enemies in your way. But you always play through the game the way the developer wants and in pretty much the same way as everyone else who plays the game. 

Fallout 3, on the other hand, lets you do most things however you want. Almost every non-player character in the game can be killed, enslaved, bargained with, or threatened. 

Hell, you can even nuke an entire town, eliminating questlines, shopkeepers, major characters, and even your own (potential) home. 

Almost every obstacle in Fallout 3 has multiple ways to deal with it, even if it is just choosing between hacking a computer or picking the lock on a door. 

You can spend your time in the game roaming the countryside saving people from slavers and super mutants, or you can just do what has to be done to advance the main storyline. 

Or you can slaughter everyone you see. Or you can sneak around dropping land mines down the back of everyone’s pants. 

Yet the game’s (original, pre-DLC) ending will be basically the same for everyone: save the day, roll credits. 

Developers constantly evolve this game type, while advances in on-rails experiences are usually mostly technical and have very little impact on the way you actually play the game. 

I had a blast playing through Modern Warfare 2 the first time. The second time, I still had fun, but it felt like watching a movie for the second time. All the same things happened in all the same ways. 

In order for games to truly evolve as their own medium, they need to allow the gamer to make choices that influence the direction the game takes instead of trying to emulate movies.