Flick wants 'dominant' Barca in vital Liga Clasico
Barca lead the Spanish top flight by four points ahead of rivals and defending champions Real Madrid with four matches remaining.
The Catalans were ousted in Europe on Tuesday by Inter Milan, ending dreams of a potential quadruple.
"We spoke about what they (the players) are thinking, what they are feeling, it's important in a group that we speak about that," Flick told a news conference Saturday.
"Everyone knows that in a Clasico you have to be on your highest level, that's what we have to do.
"We want to be active, we want to see the team (play) with intensity on the pitch, dominant as normal, and we know it's a really fantastic team from Real (against us)."
Barcelona have beaten Real Madrid in all three Clasicos played this season, beginning with a 4-0 win in La Liga at the Santiago Bernabeu in October.
The Catalans then thrashed Madrid 5-2 in the Spanish Super Cup final in January and edged them 3-2 in extra-time in the Copa del Rey final in April.
Even if Barcelona lost they would still be top of the table by a point, but Flick said his team was out to defeat Los Blancos again.
"We always go into the match to win the match, doesn't matter about the table," he added.
"(We'll) focus on this match, give everything to get three points...
"The Clasico is important for our fans, everyone, for Barca, for the players, for me, my coaching staff, it's great to play against them and compete against them."
Barcelona were handed a boost as Alejandro Balde and Marc Casado were passed fit to play on Saturday after injury.
Midfielder Casado has been missing for almost two months after a knee ligament injury, while Balde missed the last month with a hamstring problem.
Robert Lewandowski returned from injury to play as a substitute against Inter on Tuesday.
Flick said all three players were more likely to be on the bench on Sunday than to be selected as starters.
"After this match we have three more matches," noted the coach.