In addition to being a brilliant nuclear scientist, Otto Octavius is a warm, loving father with a beautiful wife named Rosalie. His family visits him for lunch at his laboratory. He supports environmental causes. He’s funny and charming, and nobody seems to mind that his job involves giant robotic tentacles.

But one day, a lab accident exposes him to radioactivity, and Dr. Octavius snaps. His calm demeanor slowly disappears. He loses his wife, family and job. He turns into a monstrous criminal. "And that’s what makes a good villain," says Avi Arad, producer of "Spider-Man 2," opening Wednesday with Alfred Molina as the tormented criminal Otto "Doc Ock" Octavius. "The best villains in film are almost normal - if you run into them in a bar, you’re not going to know you’re sitting next to someone you should run away from," Arad adds.

Superhero movies have grossed hundreds of millions of dollars at the box office, largely because of the built- in fan base that knows the mythology laid down in the vibrant hues of comic books. We know everything about superheroes - Spider-Man’s radioactive spider bite, Superman’s exploding planet Krypton, Batman’s crime-fighting vow after his parents’ murders. But just as crucial are the villains, who have their own rich and tragic back stories, which until recently didn’t always make it to the big screen.

It takes two to tangle

"You can have the greatest hero in the world, but you cannot have a good story unless you have an equally great villain," says Joe Quesada, editor in chief of Marvel Comics, one of the two major comic book companies. Marvel’s stable of heroes includes Spider-Man, the X-Men, Hulk and the Fantastic Four, while primary competitor DC Comics has Superman, Batman, Wonder Woman and the Green Lantern.

What makes a classic supervillain is something Marvel and DC have spent thousands of pages defining and developing since Superman became the first major superhero in 1938. Early on, comics drew from real life, patterning evil nemeses after Adolf Hitler. (Michael Chabon’s 2001 novel about the comics, "The Amazing Adventures of Kavalier and Clay," vividly recalls this trend.) But original supervillains appeared, in many cases, at the same time as their heroic opposites; the Joker and the Cat (later Catwoman), for example, appeared in 1939, tormenting Batman in his first appearance in comic books.

Marvel and DC bring distinctly different philosophies in creating villains, which are clearly displayed in the last two decades of blockbuster comic book movies.

Some Marvel-ous bad guys

"Some of the best villains, at least within the Marvel universe, have always been the villains who are almost identical in character structure as the hero," Quesada says. "The hero took a step to the right, and the villain took a step to the left." This fluke- of-psychology approach was especially thematic in the first "Spider-Man" movie, in which family friends Peter Parker (Tobey Maguire) and Norman Osborn (Willem Dafoe) both endured freak lab accidents, with Parker becoming the heroic Spider-Man and Osborn transforming into the evil psychopath Green Goblin.

Opposites attract

At DC, heroes and villains are designed to be arch-nemeses with no fondness for the other. "The greatest supervillains are the ones that exploit the weaknesses of our superheroes," says Dan DiDio, DC’s vice president of editorial. "Batman is a very structured life. The Joker’s about complete chaos. It forces [Batman] to think in new and varied ways to meet the challenge head-on. That’s something we’ve always done in all our storytelling."

At the box office, Marvel’s stable of heroes has dominated. The first "Spider-Man," with Dafoe as the memorably schizophrenic Green Goblin, is the top-grossing comic book film of all time, taking in $822 million box-office gross worldwide since its release in 2002. Others in the top five include 1989’s "Batman" (with Jack Nicholson’s over-the-top Joker dwarfing Michael Keaton’s dark hero), $411 million; 2003’s "X2" (with Ian McKellan’s Magneto tormenting Marvel’s X-Men), $406 million; 1995’s "Batman Forever" (with Jim Carrey mimicking Nicholson as the Riddler), $337 million; and 2000’s "X-Men," $296 million, according to Nielsen EDI.

Villainy as reality show

While movies continue to emphasize classic supervillains based on myths set decades ago, today’s comic books are radically reinventing the notions of what makes a supervillain.

Thriller novelist Brad Meltzer, writer of DC Comics’ new "Identity Crisis" series, rescues old DC villains such as Merlyn and the Calculator from obscurity and completely avoids fantastical, super-powerful bad guys such as Lex Luthor and the stone-faced, deity-like Darkseid.

"The more you can ground your villain in reality, the scarier he or she will be," says Meltzer, who focuses in "Identity Crisis" on the stalking home murder of Elongated Man’s beloved wife, Sue. "What is scary to me is not a villain who throws a car at you. What’s scary to me is I go into an empty house and I walk into the bathroom and hear a squeaky noise from behind the shower curtain. The more I can put you in that moment, the more you’re going to be terrified. That’s the most important key to writing an effective villain."

Superhero movies have generally shied away from this "Blair Witch Project"-spooky approach. In fact, until the last few years, blockbuster hero-and-villain movies have barely delved into the villains’ personal histories or motivations at all. In the 1978 "Superman" movie, Gene Hackman is funny and campy as arch-nemesis Lex Luthor, but moviegoers never learn who he is or why he hates Superman so much. Recent interpretations such as the WB’s "Smallville" have fleshed out the Luthor character, whom comic book writers have painstakingly established as a boy genius who becomes obsessed with world domination after he loses his parents in an accident. Superman’s power threatens Luthor’s control.

Lie down, Mr. Goblin

More recent movies have dug deeper into villains’ psychological histories. Dafoe, as the Green Goblin in "Spider-Man," wears a goofy costume, but fans take him seriously because of earlier scenes establishing him as Peter Parker’s friend and benefactor gone horribly wrong. McKellan’s portrayal of Magneto in both "X-Men" movies establishes him as a sympathetic character who shares hero Charles Xavier’s (Patrick Stewart) goals of liberating mutants. But Magneto’s solution is enslavement for non-mutants, and that’s where he and Xavier part ways.

"Spider-Man 2" creators believe Alfred Molina will bring the same depth of character to Doc Ock. "This [playing a villain] is tough. That’s why we look at these incredible actors to be the villain. It’s easy to let it become a cliche, so you have to underplay it," says Arad, who is also chairman of Marvel Studios. "Dafoe or Molina? They know that when we go over the top, we don’t believe them anymore. That’s what makes them not from our world. The dangerous villain is the one that seems human."