Despite it undeniably being one of the coolest things about Star Wars, Darth Vader originally just looked like a boring human being. So what changed? Concept art!
Remember how, in the beginning of the movie, Vader boards Princess Leia's (RIP, it still hurts) ship? Well, originally, he was going to fly to it, using a specially built suit. After seeing the suit, Lucas loved it so much he went, "That! Let's just make him look like that!" This helped with him turning out to be Luke's dad later — might have been a bit weird for them to excuse away the facial similarities … although they did that with Leia, so who knows?
Even after deciding he needed to be a kickbutt machine robot cyborg thing, Lucas went through a few different changes in Vader design, before landing on the awesome, scary design we all know and love.
Of course, that's not the only difference between the original Darth Vader and the one who has appeared in literally every single Star Wars film that has been made. (Yes, his charred helmet in Force Awakens counts as an appearance.) There was one huge change made, bigger than just his design …