By Alfred Hermida BBC News Online technology staff

Turning Spider-Man from a comic book character to a big screen superhero was a challenge for Hollywood special effects guru John Dykstra.

For Spider-Man the movie, he had to create a virtual character that could climb walls or leap across the rooftops of Manhattan, but still appear real to audiences.

"You have to have the natural things that occur with a human being, such as balance, posture and breathing," he told the BBC programme Go Digital.

"But if you see someone where it isn't happening, you know right away that there's something wrong - they're not real."

Expensive effects

Spider-Man is one of the most eagerly awaited movies of the year, with fans keen to see how successful it has been in bringing the American fantasy hero to life.

Maguire: From geek to superhero

It stars Tobey Maguire as the geeky science student Peter Parker, who is transformed into a one-man war against crime by the bite of a genetically engineered spider.

For the superhuman scenes in the movie, John Dykstra created a computer-generated character by filming the actor and using his movements as a reference point.

"We used a virtual character for environments where it was too risky or impractical to take our actor to," said Mr Dykstra.

But creating a life-like web-spinning superhero was not easy or cheap. Reports suggest that a third of the $80m budget for Spider-Man went on special effects.

'Huge task'

Mr Dykstra was concerned to make sure that the virtual Spider-Man appeared as real as Mr Maguire's character.

Dykstra: Humanising computer characters

"The mask was an advantage as we didn't have to recreate some of the components of the face, but at the same time it covers up one of the most expressive aspects of the human form," he said.

"So all of the things that would be expressed emotionally by the hundred or so muscles of the face have to be expressed via body posture - and it was a huge task."

Mr Dykstra is one of Hollywood's leading lights in the area of visual effects, with films like Star Wars, Star Trek: The Motion Picture, and Stuart Little to his credit.

But he does not believe that virtual characters he creates can totally replace human actors.

"The computer-generated character is another tool," he said. "For the kind of things that I work in, an actor is the way to go."

Spider-Man, directed by cult favourite Sam Raimi, is due to hit American cinema screens on 3 May, with a mid-June release in the UK.

It is expected to be a huge hit for Columbia Pictures and both Sam Raimi and Tobey Maguire have already signed up for a sequel.