I really don't know how/why this happened, it's not as if James Earl Jones' voice changed that quickly, but I think he was performing it differently in each film and they also maybe tossed some different FX at Jones' voice each time around. But he kinda almost sounds like a different guy each time around. Only direction I heard Jones' ever mention was Lucas told him in ANH that Vader was supposed to sound like an emotionless robot but, really, if you listen it's actually fairly expressive for the most part "You are PART of the REBEL ALLIANCE AND A TRAITOR". On the flipside "I find your lack of faith disturbing" is a very unemotional line-reading. Actually, now that I think about it, Vader almost seems to sound less emotional as ANH goes on (maybe it took a few line readings for JEJ to get into it.) I think his best performance in the role is ESB, just because he's basically just angrily growling every single line, and the there seems to be less FX on his voice, it doesn't sound quite as hollow. But basically Vader in ESB sounds like he's about a hair away from decapitating anyone within range with a lightsaber just for looking at him sideways and it works great. (you can really tell the difference in ESB when they drop in a few lines that weren't in the original. "Alert my Star Destroyer to prepare for my arrival." does not sound anything like his way-angry and cooler "Bring my shuttle" growl from the original. "How is that possible" in the Emperor scene sounds very different from all the other lines in that scene.) The only time, and rightly so, that he sounds anything other than angry is at the end in the "I am your father" bit and overall Jones' reading of that scene is very powerful, lots of layers. Still the growl is there "rule the galaxy as FATHER and SON" and it works because, at that point, the monster is becoming more human and hearing THAT tell Luke he's his father just warps things. In a good way. ROTJ sounds way, I dunno, hollower, deeper, kinda like he's really speaking from the bottom of a scuba tank. You could make the argument, and probably win, that this is the most emotionally complex vocal performance that Jones' has in the series but it doesn't sound all that much like he did a movie ago. The growl is basically gone, which is probably my favourite bit about in ESB. But Jones manages to put some depth in the proper places in ROTJ and Vader's performance is better for it. It may not be as scary but there's a soul in there. I'm just surprised it sounds so different from ESB. This isn't a bag on JEJ in the slightest, the guy is the bee's knees (hell, I even like how he manages to interject a quivering voice into one of his few lines in ROTS) but I've always noticed that his voice is different in each of the OT flicks. So, just for fun, what's the real-world explanation why this happened and what's the in-universe explanation as to why this happened? If you think Vader sounds exactly the same in all 3 movies, please, disregard my ramblings.