"I remember my first one when we went to the Nou Camp," former Real Madrid midfielder Xabi Alonso tells CNN, with the two La Liga rivals due to meet in Barcelona on Saturday.
"When we came onto the pitch, the whistles -- the sound of the whistles was unbelievable. So many people, 100,000 people just whistling. It was like music for us. A Colosseum."
And then you lose.
That day in 2009, Alonso and his teammates were unable to quieten the hostile Barcelona crowd, going down 1-0 to a Zlatan Ibrahimovic volley.
Alonso moved to Bayern Munich in the summer of 2014, but the Spaniard looks back on his five-year spell in Madrid with affection, having won his first ever league title and helped the club attain La Decima -- the coveted tenth Champions League trophy -- after beating city rivals Atlético 4-1 in Lisbon.
Now 35, Alonso admits he continues to pay close attention to the endeavors of his former teammates.
"I keep an eye on all my clubs: Real Sociedad, Liverpool and Real Madrid," he says.
"Of course, El Clasico especially ... The projection it has all over the world is unbelievable, and whenever you play for Real Madrid at the Nou Camp you know that you're not having the warmest welcome!"
The flying Dutchman
Another Bayern Munich player who knows all about the infamous hostility of El Clásico is former Real Madrid winger Arjen Robben.
He may have chalked up little over 50 appearances for Los Blancos, but the 32-year-old Dutch international has experienced the highs and lows of Spain's fiercest derby more than most.
"It begins a long time before the match even starts," the Dutchman tells CNN, alluding to the intense global media spotlight on the fixture.
"I have some good memories [of El Clásico], and also some not so good memories when we lost."
Notably when Robben was on the wrong end of a chastening 6-2 defeat at the Bernabéu during his final season at the club, shortly before things turned sour and he left for Bayern.
Unsurprisingly Robben prefers to look back on the good times, having added a La Liga medal to his trophy cabinet during his first season in Madrid.
Then, after wrapping up the league title with three games to spare, Robben and his teammates received a guard of honor from the Barcelona players at the Nou Camp.
"My best memory was when we had just won the Spanish league, and the first game after we'd won it we played Barcelona," he reflects.
"They had to make a pasillo and we had to go through."
Real Madrid went on to win 4-1 -- the club's biggest home league win against Barcelona since an Ivan Zamarano hat trick guided Los Blancos to a 5-0 victory in 1995.
Robben helped them on their way with a fine header, and the Dutchman grins as he recalls the squad's preparation for that famous victory, having sealed the title earlier in the week at Osasuna.
"After [winning the title] we were partying for three days, but we still won the game 4-1... I think that's my best memory of El Clásico."
"It's just amazing to play because it's two world class teams facing each other. It's a special feeling"
The enduring duo
If the two teams on show are "world class," many have suggested Barca's Lionel Messi and Real's Cristiano Ronaldo are from another planet entirely.
"I think it's extraordinary what we're seeing in the last few years from these two guys," Alonso tells CNN.
"They are, it's fair to say, well above any other player. Just in numbers, it's unbelievable what they're reaching now.
"They are different: Cristiano is so ambitious and he has the goal in his mind. He's been able to adapt, because now he's no longer the player he was at Manchester United eight years.
"Now he's in the box and, when he gets the ball, it's a goal -- with the head and with both feet.
"And I've played against Messi many times and it's so difficult to control him. In every position, he's dangerous.
"Each one has their own qualities but both -- they are great.
"For the beautiful game, to see them play each week, it's a joy to watch."