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Meet the man who stuffed as much Deus Ex into Fallout 3 as was "humanly possible"
(www.rockpapershotgun.com)
I'm not gonna give Emil too much credit when the original games had plenty of emergent gameplay and cascading choices. You could pick a lock, steal a key, blow up the door, o ther just convince characters to let you in. Such gameplay freedom is a defining trait of the tabletop RPGs that inspired videogame RPGs, so the DNA was always there.
So what sets immersive sims apart? For one, story choices are not essential, while an RPG is defined by them. In many cases, the story choices are more defined in how they relate to gameplay soup than what sort of character you are. Second, an immersive sim is far more linear than an open world RPG, with sprawling levels that allow different approaches, but a clear direction nonetheless. If linearity plots and focus on gameplay systems over narrative choice is what Emil actually took away from immersive sims, I guess that would track.