There is perhaps nothing more iconic about Star Wars than Darth Vader. In a world of glowing lightsabers and dazzling spaceships, the most iconic image remains the man behind the iron mask and mysterious black suit. Everyone remembers Vader at the height of his powers, choking to death Admirals who piss him off and going toe-to-toe in a lightsaber fight against the wise and powerful Obi-Wan Kenobi. Because of this, everyone assumes that Vader was always meant to be this iconic character, the Big Bad of the Star Wars universe.
Looking back, though, it's pretty clear Vader was more or less a henchman in the original movie. Oh, he had his badass moments, lifting Rebels up and choking them out with one hand, plus he's definitely not afraid to choke out Admiral Motti. Look back at that scene, though — Vader stops choking Motti as soon as Grand Moff Tarkin tells him to. Leia's accusation that Tarkin is holding Vader's leash, and that the Dark Lord of the Sith is more akin to Tarkin's attack dog seems awfully accurate.
Vader is apparently the guy who has to process the paperwork when Tarkin orders Leia terminated (keep in mind she was merely scheduled to be executed—Vader was a paper pusher). And, of course, Vader risks his own life to fight the Rebel Starfighters at the end. It doesn't make much sense for a major leader of the Empire, second only to the Emperor, to put himself in a position where a stray laser bolt could immediately kill him. It does make sense for Tarkin's enforcer to do that, though, getting his hands dirty while Tarkin gives the orders from relative safety.
Still don't believe it? Check this out: when George Lucas was plotting Splinter of the Mind's Eye, a low-budget sequel to Star Wars, with author Alan Dean Foster, he admitted to Foster that he thought Vader wasn't a strong villain at all! And this isn't just Lucas being his usual wrong self — at the time, Vader simply wasn't strong.