If Dumbledore is the epitome of goodness in Harry Potter, then we all know who the model of evil must be: "Lord" Voldemort. Most of what we know about Voldemort at this point, we get from Book 2, Harry Potter and the Chamber of Secrets. And we find out a heck of a lot more in Book 6, Harry Potter and the Half-Blood Prince. But in Goblet of Fire, Voldemort is mostly just a symbol of pure evil. He doesn't have a lot of character depth yet. Everything that Voldemort does in Goblet of Fire is a means to an end, as he struggles to regain his body.
In the early stages of the book, Voldemort must rely entirely on the help of Peter Pettigrew, a.k.a. Wormtail, who has fled England now that his secret identity has been revealed. Wormtail uses his rat form to travel to the forests of Albania, where he found Voldemort as a spirit. Using a spell based on unicorn blood, Wormtail gives Voldemort a body. But it is not a good body. Voldemort no longer looks at all human – he's more of a tiny, revolting, scaly, red thing. Oh, and he has to be fed every few hours or he'll die. In order to come back fully, Voldemort needs Wormtail's help to make another potion, one that involves the bones of his father, Tom Riddle. This is what brings Voldemort back to Little Hangleton, England, where he sits in the old Riddle house waiting for everything to come together.
(In the first chapter, by the way, we get a sense of Voldemort's awful sense of humor. He tells Wormtail, "I will allow you to perform an essential task for me, one that many of my followers would give their right hands to perform" (1.71). Get it? Get it? Right hand? Mwah-ha-ha-ha-haaa. Yes, Voldemort is referring to the ritual when Wormtail has to cut off his right hand to bring his master back in Chapter 34 – a ritual Wormtail doesn't know about yet. Oh the humor, it has us in stitches.)
While in England, Voldemort gets in touch with a lost lamb of his: Barty Crouch, Jr., Voldemort's "faithful servant" (1.65). Barty Crouch, Jr. disguises himself as a teacher at Hogwarts (Mad-Eye Moody) and arranges Harry to travel by Portkey straight to the graveyard at Little Hangleton where Tom Riddle's bones are lying. With the help of his father's bones, Harry's blood, and Wormtail's severed hand, Voldemort is brought fully back to life.
Once Voldemort actually gets his body once more, we get to see a bit more of his "playful" side. Voldemort gives Harry back his wand and decides to set up a wizard duel between the two of them. He forces Harry to bow in front of him, and then starts using the Cruciatus Curse to cause Harry pain. Voldemort is torturing Harry for fun, yes, but he also has a goal in mind: he wants to impress his followers so that they're reassured about his regained power. And he wants to frighten the former Death Eaters into following his commands once again. Having been driven out of power when he tried to kill Harry thirteen years before, Voldemort wants to kill the boy now to save face. But when Harry manages to escape through some fantastic good luck (again), things must have been a bit awkward in the Little Hangleton graveyard.
Like Dumbledore, Voldemort seems to know a great deal more than most people in this novel. But Voldemort's performance of all-knowing superiority is a lot less successful than Dumbledore's. Voldemort may like to bully people, but that doesn't mean he's actually as strong as he pretends to be. We get to see the cracks in his façade when Harry and Voldemort's wands meet. Voldemort's "red eyes [go] wide with astonishment" (34.31) when the Priori Incantatem starts, and Voldemort even looks "astonished, and almost fearful" (34.37) as the ghosts of his victims begin to come out of his wand. Voldemort has weaknesses, even if Harry doesn't know exactly what they are yet.
Something else that neither Harry nor Voldemort seem entirely to understand – though Voldemort catches on quick in Book 5 – is the curse scar. Harry's lightning bolt-shaped scar is what connects him psychically to Voldemort. Whenever Voldemort is feeling especially angry, Harry gets flashes of it, as when Voldemort murders Frank Bryce and when he casts the Cruciatus Curse on Wormtail. So, through Harry, we can get signs of how Voldemort behaves in his off hours when he's just hanging out, and it's not a pretty picture.