What e3 demoes aren't scripted?
Most of the time people are just pretending to play the game as a video rolls bye. And even the ones that are played, they essentially have cheat codes on like god mode. An e3 demo is designed to do multiple things like showing of the premise and gameplay.
Example being the last of us demo, a lot of the things showcased in that demo aren't in the game. You can't wrestle away guns, enemies won't pounce on you, but for the most part a lot of things shown were still in the game.
The same can be said about watch dogs, sure it doesn't visually look like what was shown, but the gameplay is intact. The tone has changed, but after watching interviews, the team has stated they had the idea, before they had created a story. Also, watch dogs demo was in an odd place of it was them showing what they hoped they could do with next gen.
Games change and sometimes a dev team can't deliver everything they wanted. Just go into e3 and say this is what the game will probably be like, if what you are being shown is not linear, and has some degree of openess.