The two Spanish giants are fierce rivals on the pitch and that spilled out into open animosity between their respective players.
And although Real goalkeeper Casillas acknowledges that it was a problem a year ago, he insists it's all been cleared up now.
"It would have been a problem if we'd been playing the Euros last year but now this season we're fine and we're going to play," said the Spain captain.
"Last year Barcelona and Real Madrid players were not on good terms but now this is a strong motivation for all of us, it doesn't matter which club.
"Of course you're going to get disagreements but you must look on the bright side. We have great relationships, we play for our teams but now we're playing for the national side."
Barcelona midfielder Iniesta said it would be futile to rake up old dirt.
"We don't need to elaborate on this topic, it wouldn't make sense to talk about it now, it would be stupid," he said.
"We're all here to defend the same idea which is above all to play the best Euros we can.
"The atmosphere in the camp and our relationships are perfect, everything is going well."
Even coach Del Bosque was at pains to insist it was all water under the bridge.
"This is the past, it's not a current topic. The atmosphere in the squad is great, they're working together," he said.
"This happens in a group, sometimes you have rivalry but we've got over it."
Reigning champions Spain open their Euro campaign against Italy in Group C on Sunday.
They met at the quarter-final stage of this competition four years ago with Spain squeaking through on penalties.
But back in August the world champions were upset 2-1 in Bari.
"Italy have changed, they've changed players, they've changed coach and their style so this will be a fight for us," said Del Bosque, referring to the differences since the teams' meeting four years ago.
"We want to be the ones who have the initiative and play with dynamism. We have a strong team, we're the world champions.
"We met in Bari, they're a very strong team and we'll have to put everything into it."
Italy played the entire qualifying campaign for this tournament under Cesare Prandelli with a regular back four but the coach is almost certain to switch to a system with three centre-halves against Spain, with Roma midfielder Daniele De Rossi set to move back into a libero role.
And Del Bosque admits that has caused some uncertainty for him.
"We have watched Italy and Prandelli's style of play. We don't know what he will play, we can't say," admitted the Spaniard.
"It has changed but De Rossi has already played in this position, it's the coach's decision and we have to react to it, but it's not that important.
"Our own task is to prepare tactically, mentally and physically in the best way we can. Our own style of play is the most important thing."
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