HOLLYWOOD — This modern era of moviemaking has plenty of peculiar challenges for actors. On green-screen sets, for instance, they have to watch a pingpong ball hanging from a string and convince the camera that they are actually staring down some magical beastie. But for the actors in "Avatar," the toughest task may have been speaking words invented by Paul R. Frommer, a professor at the University of Southern California.

Frommer, a linguist, was hired by "Avatar" writer-director James Cameron to create a functioning language for the tribe of 10-foot-tall blue beings who inhabit Pandora, the otherworldly setting for the film's conflict. Frommer tackled the project with glee — "How often do you get an opportunity like this?" — but the actors who had to bend their tongues around the invented vocabulary and syntax were slightly less charmed by the experience.

"Oh, it was so hard, and I was really concerned about it," said Zoe Saldana, who plays Neytiri in the science fiction adventure that opens Dec. 18. "I didn't think I could get through it. I'm not good with languages. All the actors, we worked together. It was the only way."

Frommer has spent four years laboring on the language of the Na'vi tribe, and his work will not end soon. He plans to expand the language until he's, well, blue in the face.

"I'm still working and I hope that the language will have a life of its own," Frommer said. "For one thing, I'm hoping there will be prequels and sequels to the film, which means more language will be needed. I spent three weeks in May, too, working on the video game for Ubisoft, which is the name of a French company. That's not a French word, though; I don't know where they got Ubisoft."

Frommer is clearly delighted by his unexpected excursion into the Hollywood dream factory, which has the buttoned-down academic working with movie stars and hobnobbing with Oscar-winner Cameron.

Sitting on a concrete bench near the bustling center of USC's campus recently, he recounted his Tinseltown labors with verve; the hint of disappointment was when he explained that his language was limited by the earthling larynxes of cast members.

"The constraint, of course, is that the language I created had to be spoken by humans," Frommer said. "I could have let my imagination run wild and come up with all sorts of weird sounds, but I was limited by what a human actor could actually do."

Including the scripts for the film and the video game, Frommer has made up more than 1,000 Na'vi words, as well as all the language's rules.

"I'm adding to that all the time," said Frommer, who says he would like to see the new tongue catch on in the way "Star Trek's" Klingon has.

"Oh, I'm very aware of Klingon," Frommer said the way a sports coach might analyze a rival with a long winning tradition. "It was created by a linguist (named Marc Okrand), and it is very, very well put together. I actually once developed a problem for students in analysis using data from Klingon. When I started working on this, though, I deliberately did not look at Klingon so I wouldn't be unconsciously influenced by it."

To some ears, Klingon sounds like a cross between Russian and crawfish, but Na'vi is far more gentle on the ear.

Frommer said: "Cameron wanted something melodious and musical, something that would sound strange and alien but smooth and appealing."

Frommer is a linguist and got his doctorate at USC. Then he left academia for the business world. "I really wanted to teach, though, and came back," he said.

He ended up on the faculty of the Center for Management Communication at the Marshall School of Business and is teaching clinical management communication. But deep down, he said, his true love is for language and linguistics.

When "Avatar" producer Jon Landau approached the linguistics department at USC about creating an extraterrestrial tongue, the request quickly found its way to Frommer, who had once collaborated on a workbook that collected data from 30 languages.

"The e-mail that came my way said they were looking for someone who could create an alien language for a major motion picture directed by James Cameron, but the name of the project at that time was Project 880," Frommer said. "As soon as I saw that e-mail, I pounced on it."

Frommer didn't start from scratch; Cameron had come up with about three dozen words of Na'vi, but most were character names.

"It gave me a sense of the sound that he was looking for, and then I expanded it. Given these sounds and the possible combinations, what further structure could I bring to the sound to make it interesting?" Frommer said. "That was the starting point. Probably the most exotic thing I added were ejectives, which are these sorts of popping sounds that are found in different languages from around the world. It's found in Native American languages and in parts of Africa and in Central Asia, the Caucasus."

Then came the heavy lifting — nailing down the sound system, word construction, syntax — and Frommer immersed himself in the thousands of decisions required, many of them deciding what goes in and what comes out. The Na'vi language, for instance, does not have the sounds buh, duh, guh, chu and shu. By restricting the sounds, Frommer said, the shape of the language becomes distinct.

"If you allow everything and the kitchen sink, you get a mishmash — it sounds like gibberish," Frommer said. "An analogy is cooking and deciding how you are going to spice up a certain dish. If you put everything you have on the shelf, you get a mess. If you are judicious, you get something good. In language, sometimes things are defined by the absences."

The finished product sounds, to some ears, vaguely Polynesian. Others hear the rhythms of African languages.

"Someone said it sounded German to them. Someone else told me Japanese. and I think that's good," Frommer said. "If everyone were saying one single language, then it would be bad."

Frommer worked with the actors and dialect coach Carla Meyer, whose credits include the "Pirates of the Caribbean" films, "Angels & Demons," "Erin Brockovich," and "Air Force One," in which she helped Gary Oldman shape his hijacker's Eastern European accent. Frommer was impressed with the actors' intensity of focus.

"I was surprised they all did very well, and it gave me hope, too, that other people will try to learn it and speak it," Frommer said. "I'm excited because there is going to be a Pandorapedia online and a lot of material for people to learn more about the planet. There's this incredible devotion to detail. It's been fascinating to me. It's almost academic in its approach."

Frommer finds himself walking the campus and talking to himself in Na'vi.

He has attempted poetry, too. It wouldn't be surprising if some of his couplets were forlorn — it's lonely being the only person speaking a language.

"I just wish," he said, "that I had someone to talk to."