Stallone's "arthritic" action heroes roll their tanks through Cannes
* Being good action star not a matter of muscles -Stallone
* "We are children with arthritis" says "Rambo" star
* Third "Expendables" film set to open this summer
CANNES, France, May 18 (Reuters) - Tanks rolled down the beachfront in Cannes on Sunday, as Sylvester Stallone led an invasion of action stars "with arthritis" to publicise "The Expendables 3".
Stallone, Arnold Schwarzenegger and Mel Gibson waved at fans from three tanks that drove slowly down the main boulevard in the French Riviera town where the international film festival is in its fifth day.
The film, which is not in competition, also features Harrison Ford, Antonio Banderas, Jason Statham, Dolph Lundgren and Wesley Snipes and the cast was asked by reporters to describe just what makes a good action star.
"You have to be incredibly intelligent, handsome, sexy, good teeth, high IQ, a lot of hair..." Stallone joked.
"What makes the audience like a person, it's not a matter of muscles," he told a press conference.
"There's something that's almost intangible and I don't know what it is, but I know it's very rare because there have only been about 15 action heroes, real serious ones, in history."
Stallone, one of the film's writers, said it took three movies to get the "Expendables" formula just right, judging the first as being too violent and the second too soft.
"I believe we finally got it right in the third one - kind of like marriage."
Stallone acknowledged that he and the rest of the cast belonged to a "different generation", but still had the energy and drive to make an exciting, contemporary film.
"We're all like very adult children," Stallone said. "We are children with arthritis. We are young forever."
The third instalment of the franchise sees Barney Ross (Stallone) bringing in new blood to his team of mercenaries to take on Conrad Stonebanks (Gibson), a former comrade now seeking to eradicate the group.
One of the newbies is played by Kelsey Grammar, best known for his role as the pompous psychiatrist on the television sitcom, "Frasier," which ran for over a decade on NBC before ending in 2004.
"Although I feel like I'm new to action films, my star is just rising," Grammar joked.
"Expendables 3" wasn't the only big-budget franchise trying to grab attention at the festival.
The cast of the latest "Hunger Games" movie, "Mockingjay Part 1", walked a red carpet for photographers on Saturday night, advertising the film due for release in November.
When Jennifer Lawrence, who plays protagonist Katniss Everdeen, was asked why there should be so much fuss about a film not even showing at the festival, the actress was quick with a response: "Because so much money went in to it."
"I'm kidding!" she added. "Because they're great movies and we have an amazing audience and a great following and ... Whoops!" (Additional reporting by Mike Davidson and Rollo Ross; Editing by Michael Roddy and Robin Pomeroy)