The most heated film battles often take place behind the scenes instead of on the big screen.

It's the struggle between those who rate movies and those who make movies — and it can make or break a film's box-office success.

The Motion Picture Association of America determines the ratings for all movies, sometimes baffling audiences and critics as to why one film gets one rating while another seemingly similar flick gets a totally different rating.

"The reason for the controversy at the ratings board is not just how much money is riding on those ratings, it's how mysterious they are," movie reviewer Michael Medved said. "Nobody can figure out why something gets a PG-13 rating, something else gets an R-rating, something else gets an NC-17. It's the most arbitrary process in the world."

While everyone knows the group counts the number of nude shots and curse words, the actual people who make up the MPAA are a surprisingly mysterious lot.

"There are six anonymous people," Medved said. "I sort of envision them like the ancient Spanish inquisition. Sort of wearing hoods on their heads so you don't know the identities with little eyeholes for them to watch the movies. You can ask people, they won't tell you who these people are."

The process that assigns movies a G, PG, PG-13, R or NC-17 rating even has Hollywood headliners shaking in their Prada boots.

"I think it's scary, really scary, that there's a group of individuals in this country who have the power to keep something from getting out there," said actor James Van Der Beek.

The Dawson's Creek star's latest movie, The Rules of Attraction, originally was rated NC-17, which is considered the kiss of death at the box office. It carries the stigma of the old X-rating and many theaters refuse to even show NC-17 movies.

"It really doesn't make any sense, it seems so silly you know," Van Der Beek said. "You cut a couple of frames out here and all of a sudden, oh, now its OK. To me it seems like a bunch of people trying to justify their existence."

Rules was eventually trimmed to an R rating and still didn't attract much business at the box office.

Neither did the Antonio Banderas, Lucy Liu picture, Ballistic: Ecks vs. Sever, which received an R rating as well. But director Wych Kaosayananda was baffled because he said he'd been shooting a PG-13 movie all along.

"We tried really hard in shooting it to keep it from being an R," he said. "I watched a lot of movies that were PG-13 and seeing what they got away with. We wrote out all the bad language, there was no nudity, and there's really even no blood in the movie, but what can you do?"

MPAA chief Jack Valenti said his group tries to reflect what's considered kosher by mainstream society.

"We try to move with the acceptability level in the community," he said. "This is all very subjective, we are not dealing with geometry or Boyle's law of gases."

But critics like Medved said the board's standards pan out to be just plain arbitrary.

"It seems to me that the standards that they're using, which is counting the number of harsh words or the number of graphic nipple shots or rear end shots, to determine whether it gets R or PG-13, makes no sense," Medved said. "Because there's material that has very, very adult themes, the kind of themes that parents would be concerned about.

"It shouldn't only be how much nudity you have or how many bad words you're able to count. It should be what are the underlying messages."

But Valenti said it's not fair to criticize the MPAA when even society can't decide what crosses the line.

"We're dealing with highly murky, illuminated subjects, because subjectivity means indecision," he said. "That's why you have disagreement."