Every day when Guillermo del Toro went to meetings during the making of Pacific Rim, the visual effects crew found itself in a room, not with an award-winning director, but a tirelessly passionate fanboy.

"He was giddy about the whole process," says Alex Jaeger, visual effects art director on the film del Toro waited 40 years to make. "He was like a 12-year-old boy, constantly going, 'Show me more!"'

That childlike enthusiasm immediately stirs up thoughts of another fanboy-turned-director who took years to get his dream project rolling - Peter Jackson.

Had the studio politics worked out differently, del Toro would still be along for that ride, having co-written the trilogy's screenplay but dropping out as director because of contractual issues.

Instead, he's sitting at the Four Seasons Hotel in San Francisco waxing lyrical about monsters, "robot porn" and the film he has wanted to make since he was a young boy growing up in Mexico.

Hearing his diehard devotion, it's no surprise del Toro was overflowing with excitement every minute of production.

"Put it this way," he muses. "You can see when a film-maker gets high on his own supply. I get high on my own supply - I get off with the monsters.

"For me the era of giant robot movies started when I was 8 years old and I've been waiting patiently to get this made."

In a nutshell, the sci-fi tale is a battle between jaegers (giant robots) and kaijus (giant monsters).

War breaks out after a breach in the Pacific Ocean floor unleashes ferocious 76m-tall kaijus, which wreak havoc everywhere from the US to the Sydney Opera House. In response, mankind unites to build jaegers, operated simultaneously by two human pilots who sync their brains to handle the mental and physical load of the machines.

Del Toro, whose credits include Pan's Labyrinth and Hellboy, is fiercely adamant that this is not simply computer animation.

"Calling it that would be like calling a painter's work 'brushwork'."

Creating the kaiju was like "Christmas in creature model heaven" for digital model/creature supervisor Paul Giacoppo from Industrial Light and Magic.

America's best-known special effects house since George Lucas founded it to make Star Wars, ILM has made some big creatures in its time, including for that other giant robot series, Transformers. But the scale of the kaiju was a creative test.

"You know when you get out of the bath and take a little water with you? These things are so big that they get out of the ocean and take half the bay with them," says Giacoppo.

He adds that although dinosaur and monster movies often draw inspiration from large animals, the scale of the kaijus and jaegers meant there was nothing comparable in film or reality. Del Toro was reluctant to take the easier route of using motion capture technology, so the team instead turned to everything from rocky cliff faces to sea slugs to create monsters such as Knifehead.

The jaegers were based on World War II machinery, each sporting the colours and insignia of their countries. But although there's been promotional artwork of New Zealand's "Kaiju Crush" it wasn't used for the movie.

Designers worked from the inside out on the jaegers, beginning with the pistons, engines, levers and inner detailing, which del Toro enthusiastically refers to as "robot porn".

Meanwhile in Toronto, a 20-tonne Conn-pod (the cockpit from where the pilots operate the jaegers) was built, the interior redressed to suit each robot.

Tom Cruise was initially set to play the jaegers' boss, Lieutenant-Commander Stacker Pentecost, but was unable to commit, leading del Toro to cast Luther and The Wire star Idris Elba. But it wasn't until the 48-year-old director met Luther creator Neil Cross in New Zealand that he was able to cast Elba.

That's not Pacific Rim's only Kiwi link - Wellingtonian Kate Hawley worked as costume designer, collaborating with del Toro on the pilots' 11kg speciality suits.

"I had the privilege of being part of the concept," says Hawley, who also worked on The Hobbit. "It was a lovely opportunity to find ways to echo the characters' personalities as well as their backgrounds."

Sons of Anarchy star Charlie Hunnam, who plays pilot Raleigh Becket, is quick to point out how uncomfortable the bulky outfits were. "Unless you have a fetish for wearing a suit of armour while working out on an elliptical machine with 200 gallons of water being poured on you a minute, there was nothing you could do to prepare for filming in the Conn-pod!"

The final challenge in the project was the question mark hanging over whether the film should be 3D.

Del Toro initially took a crash course in 3D from James Cameron, but budget restraints and limited production days prevented him from using the medium.

When Warner Brothers later wanted to convert the film, he feared they might "over-3D-ise" it and issued three demands - more money, a test of the most difficult shots and more time, so he could be involved throughout the process.

The end result?

"The thing that converted was me, not the film. I'm a f***ing crazy fan of 3D now - for this genre. I don't want to do Pride and Prejudice in 3D but in this movie I adore it."

As for a certain other big monster-filled 3D film releasing this year, del Toro's lips are sealed.

"I am under huge contract not to discuss any details of The Hobbit," he says with a grin, before glancing out the hotel window. "And there's a sniper out there watching me right now."

Who: Guillermo del Toro
What: Pacific Rim
When: Opens at cinemas today

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- TimeOut