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The build-up in the Spanish media to Saturday's match between Real Madrid and Barcelona at the Santiago Bernabeu Stadium is all-consuming. The onslaught—across television, radio and newspapers—lasts for two weeks. In between the endless commentary comparing the merits of Leo Messi, Cristiano Ronaldo and the teams' coaching staffs, there is analysis of the referee who has been chosen for the match.

The man in the spotlight this time around is a 34-year-old from Murcia called Jose Maria Sanchez Martinez. His biographical details and track record have been pored over like the case histories of a Supreme Court appointee. Unlike top-flight leagues in most other countries, La Liga lists the referee in advance of a fixture, not afterwards. This immediately opens Sanchez Martinez up to scrutiny.

Cesar Muniz Fernandez retired from referreeing in La Liga in 2014. He took charge of three Barca-Madrid matches during his career, all of them at the Bernabeu, including a 2-1 defeat for Real Madrid a first-leg Copa del Rey encounter in January 2012, a match widely remembered for Pepe slyly stamping on Messi's hand as the Barcelona forward lay on the ground.

Muniz Fernandez says he avoided looking at press previews in the run-up to the Clasico: "It's very important when you referee the match not to watch television, the Internet, to read the newspapers. You have to forget about it. You have to isolate yourself when you're on the margins of so much media pressure. You have to concentrate on the match. You must have a clear mind if you're going to make the correct decisions."

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According to , 650 million people will watch the Clasico on television, added to approximately 81,000 fans that will be present in the Bernabeu. That's a lot of eyeballs, although Muniz Fernandez says the nerves didn't get to him: "You get used to the pressure of referring in La Liga. There is a lot of pressure for every match. I was very happy because it's very important to be selected to referee this match. It's a special occasion. The whole world is watching this match on television. It's an honour to referee it."

This is the second time Sanchez Martinez will take charge of a Clasico. He refereed the second leg of last summer's Spanish Super Cup final in which Real Madrid won 2-0, securing the cup on aggregate 5-1. Since being promoted to the premier division at the end of the 2014-2015 season, Sanchez Martinez has referreed Barcelona eight times, in which they've won five matches, per Diario AS. He's overseen Real Madrid the same number of times, but they've a higher win ratio over these games, with seven wins and one draw.

The statistical difference has opened him up to charges of being biased towards Real Madrid from the Catalan-based press with El Mundo Deportivo (in Spanish), for example, running a profile of him with the headline: "Sanchez Martinez, a bad memory referee."

In the article, he's criticised because he "treated Real Madrid favourably this year," maintaining, among a string of perceived grievances, that he gave Real Madrid two penalties that "should not have been granted" in a league match against Alaves. The article adds: "he's not remembered pleasantly by los azulgrana (Barcelona's fans)."

"The real show is the players," said Rafa Guerrero, a former linesman in La Liga, "but if a journalist starts a week before to examine the referee and says, 'This is the guy who damaged Barca last year' or this kind of information, it's terrible because fans read this and go to the stadium influenced by what they read. This generates violence. I have experienced it; I have had to get protection at my house. My boy came home from school one day because the other children said that his father was a robber."

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The problem of corruption and match-fixing—or what is referred to in shorthand in Spain as "maletin," the "little suitcase" of cash bribes—exacerbates the referee's predicament. Javier Tebas, president of La Liga, admitted in 2013 that between eight and 10 matches in Spain's two top divisions were fixed each season, per BBC. This feeds into a culture of suspicion in the eyes of La Liga fans when it comes to the referee's performance.

"In northern European nations—in the English Premier League and in Germany and Holland—there is a supposition of fair play," said John Carlin, author of White Angels: Beckham, Real Madrid & The New Football—The Inside Story. "The referee will do his best, and he will not be biased. It is possibly disingenuous, but it underpins the whole spectacle. It's a widely shared conviction that the referees are doing an honest job.

"In Spain, as in Latin American countries such as Argentina, there is far greater scepticism and doubt regarding the referee's impartiality. They are societies in which people tend to be more distrusting of their fellow men and women. There is a perception—and I stress it's just a perception—that people will try and cheat you and con you if they can get away with it. There is much more of a perception that referees can be bought, one way or another.

"What I heard when I was researching my book on Real Madrid in 2003-2004 was that it was standard for referees to arrive in the city where they were playing a game and the host club would leave them gifts for their wives in their hotel room, or give them spending money or indeed in some cases take them to nightclubs and furnish them with prostitutes.

"This is what I heard within the corridors of Spanish clubs some years ago. Whether it is going on now, I don't know. More to the point, whether it is true or not people generally in Spanish football believe those sorts of rumours to be true. If you were to say this happened in an English club, the reaction of the average fan would be to doubt it or be deeply shocked. The reaction of the average Spanish fan would be: 'We're hardly surprised.'"

There is an acceptance, too, that the players on the pitch are not always upstanding. They will try to bend the rules if they can get away with it. "In Spain, a lot of players are more 'picaro,' cunning," Guerrero said. "They play with their hands or fall down on purpose. In other European countries, they would be punished, but here it is more tolerated. Players act. They try to deceive the referee. It's to do with their football education."

Of the players who will take part in Saturday's Clasico, Guerrero identifies Real Madrid's captain Sergio Ramos as being particularly skilled in the arts of the picaro, a figure celebrated in Renaissance literature for his wiles, for getting by on his wits.

Ramos holds the record in La Liga for being sent off more than any other player, 22 times during his 12-year career with Real Madrid, as per Marca (in Spanish). Guerrero says Ramos will feign injury to get an opposition player in trouble: "Sergio Ramos always says: "He did it, ref.'"

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Spanish football fans are provincial by nature, especially when it comes to the Barcelona vs. Real Madrid rivalry—almost two-thirds of the country supports either Real Madrid or Barcelona as their chosen team, according to a 2014 study by Centro Investigaciones Sociologicas (h/t Goal.com, in Spanish).

The teams' fans will take it personally if Sanchez Martinez makes a dubious call against their side on Saturday if, say, he awards Real Madrid a penalty or hands out Ramos another red card. It will be as if he has brought local dishonour to them, so he will incur their wrath in that very Spanish way.

"What I long ago identified as being the favourite sport of the Spanish, way ahead of football or bullfighting, is indignation," Carlin said. "The Spanish love to be indignant. My mum's Spanish. I lived in Spain for 15 years. I've had that perception roundly confirmed at every turn.

"There are these hand gestures that accompany the indignation that people express at any available opportunity, be it in the private sphere, the political sphere and certainly by God in the all-important football sphere.

"Indignation is a very pleasurable emotion. Externally, people grimace, make loud noises and express rage. Internally, it's very satisfying because it always springs from a profound conviction of your own moral rightness. Indignation and self-righteousness are two sides of the same coin.

"There's a feeling of being morally superior to the targets of your indignation. In a football stadium, there's probably nothing that generates more passion and noise—with the possible exception of a winning goal in the last minute – than an opportunity to vent indignation at a referee or a rival player. A referee is incredibly handy—you can go after him all day long."

All quotes and information obtained firsthand unless otherwise indicated.

Follow Richard on Twitter: @Richard_Fitz