The first rule of authoring interactive fiction is--- Make the fiction interactive. Obviously, you say. More specifically, the IF author should not, at any point, force the reader to do anything. The reader/actor (I'll try to avoid the gaming term, player, for the reader) should decide the course that they take as a character in the fictional world. This is not to say that the author cannot guide the reader/actor, but the guiding must be done in an artful fashion and not heavy handedly. Give the reader more than one option in any given situation. Make the reader's interaction significant. Don't do this:
>walk down hallway
You go to the end of the hallway, you see a door.
>You open the door, see Lenny, and fire the gun killing him.
If the reader wants to shoot Lenny, let them make the decision, even if that means sacrificing the story you have in mind. The point here is don't grab the keyboard and type actions in for the reader/actor. All actions should be taken by the actor/reader.