They came from the far reaches of the Amazon, traveling in small boats and canoes for up to three days to discuss their fate. James Cameron, the Hollywood titan, stood before them with orange warrior streaks painted on his face, comparing the threats on their lands to a snake eating its prey.

The Arara tribe, who live along the Xingu River in Brazil, are among the indigenous peoples who oppose a proposed dam.

“The snake kills by squeezing very slowly,” Mr. Cameron said to more than 70 indigenous people, some holding spears and bows and arrows, under a tree here along the Xingu River. “This is how the civilized world slowly, slowly pushes into the forest and takes away the world that used to be,” he added.

As if to underscore the point, seconds later a poisonous green snake fell out of a tree, just feet from where Mr. Cameron’s wife sat on a log. Screams rang out. Villagers scattered. The snake was killed. Then indigenous leaders set off on a dance of appreciation, ending at the boat that took Mr. Cameron away. All the while, Mr. Cameron danced haltingly, shaking a spear, a chief’s feathery yellow and white headdress atop his head.

In the 15 years since he wrote the script for “Avatar,” his epic tale of greed versus nature, Mr. Cameron said, he had become an avid environmentalist. But he said that until his trip to the Brazilian Amazon last month, his advocacy was mostly limited to the environmentally responsible way he tried to live his life: solar and wind energy power his Santa Barbara home, he said, and he and his wife drive hybrid vehicles and do their own organic gardening.

“Avatar” — and its nearly $2.7 billion in global tickets sales — has changed all that, flooding Mr. Cameron with kudos for helping to “emotionalize” environmental issues and pleas to get more involved.

Now, Mr. Cameron said, he has been spurred to action, to speak out against the looming environmental destruction endangering indigenous groups around the world — a cause that is fueling his inner rage and inspiring his work on an “Avatar” sequel.

“Any direct experience that I have with indigenous peoples and their plights may feed into the nature of the story I choose to tell,” he said. “In fact, it almost certainly will.” Referring to his Amazon trip, he added, “It just makes me madder.”

Mr. Cameron is so fired up, in fact, that he said he was planning to go back to the Amazon this week, this time with Sigourney Weaver and at least another member of the “Avatar” cast in tow.

The focus is the huge Belo Monte dam planned by the Brazilian government. It would be the third largest in the world, and environmentalists say it would flood hundreds of square miles of the Amazon and dry up a 60-mile stretch of the Xingu River, devastating the indigenous communities that live along it. For years the project was on the shelf, but the government now plans to hold an April 20 auction to award contracts for its construction.

Stopping the dam has become a fresh personal crusade for the director, who came here as indigenous leaders from 13 tribes held a special council to discuss their last-ditch options. It was Mr. Cameron’s first visit to the Amazon, he said, even though he based the fictional planet in “Avatar” on Amazon rain forests. Still, he found the real-life similarities to the themes in his movie undeniable.

The dam is a “quintessential example of the type of thing we are showing in ‘Avatar’ — the collision of a technological civilization’s vision for progress at the expense of the natural world and the cultures of the indigenous people that live there,” he said.

Mr. Cameron said that he was writing a letter to President Luiz Inácio Lula da Silva urging him to reconsider the dam and that he would press for a meeting with the president. “They need to listen to these people here,” he said.

Mr. Cameron, 55, first encountered the cause in February, after being presented with a letter from advocacy organizations and Native American groups saying they wanted Mr. Cameron to highlight “the real Pandoras in the world,” referring to the lush world under assault in his movie.

Atossa Soltani, executive director of Amazon Watch, who accompanied him on his trip last month, said Mr. Cameron lit up at the idea of learning more, saying he had grown up in the Canadian woods and had even logged thousands of hours underwater exploring the world’s oceans.

As for Mr. Cameron’s Amazon adventure, it got off to a rocky start. The boat he traveled to the village in flooded when a hose became disconnected. Mr. Cameron chipped in, grabbing a plastic bucket to help bail for a few hours in the searing midday heat, he and others on the boat said.

Many of the indigenous leaders he was planning to meet with had never heard of him before, much less seen his movie. All they knew was that “a powerful ally” would be attending their gathering, Ms. Soltani said.

So, the night before Mr. Cameron and his wife, Suzy Amis, arrived with three bodyguards, a dozen or so villagers gathered in the house of José Carlos Arara, the chief of the Arara tribe here, to watch a DVD of “Avatar.”

“What happens in the film is what is happening here,” said Chief Arara, 30.

The morning after Mr. Cameron’s party arrived in the village, Chief Arara led them on a walk through the rain forest. Mr. Cameron, almost mirroring the enraptured scientists in his movie, was calm but wide-eyed, peppering the chief with questions about the local fauna and flora and traditional indigenous ways. In seconds, the chief showed how he could fashion ankle braces from leaves to help him scale an açaí tree.

The leaders then invited Mr. Cameron to participate in their meeting. He sat at a small wooden school desk as they made speeches condemning the impending dam and the Brazilian government. Mr. Cameron seemed to tear up when some leaders said they would be willing to die to stop the dam.

Finally, Mr. Cameron was asked to speak. He stood and complimented the leaders on their unity, saying they needed to fight off efforts by the government to divide them and weaken their resistance.

“That is what can stop the snake; that is what can stop the dam,” he said.

A rush of applause swept through the crowd. When the real snake fell from the tree, the director seemed unfazed. After clearing it away, indigenous leaders thanked him with gifts. One gave him a spear, another a black and red necklace of seeds. A third, Chief Jaguar from the Kaiapo nation, one of Brazil’s most respected, gave him his headdress before the dances in Mr. Cameron’s honor began.

“It’s not like there is any pressure on me or anything,” he said, half-joking, moments before boarding the boat. “These people really are looking for me to do something about their situation. We have to try to stop this dam. Their whole way of life, their society as they know it, depends on it.”