The match comes at a critical juncture after defeat by Paris Saint-Germain in the group stages for a Barca side who were a few minutes from reaching their first Champions League final in a decade in Flick's first year in charge last season.
The German coach acknowledged his recent emotional outbursts, including being sent off during the Spanish champions' dramatic 2-1 win over Girona in LaLiga on Saturday when Ronald Araujo's stoppage-time strike secured victory.
The 60-year-old Flick, who was shown a red card for allegedly protesting against refereeing decisions in the dying minutes of the match, said he had been overwhelmed by emotion.
"I'm not more nervous, maybe my emotions are not the same as before," he told a press conference on Monday.
"I remember when I was coaching Bayern Munich, there were images of us winning 8-2 against Barca, and I never smiled with the eight goals.
"Now I have more emotions; this club have completely changed me. What I can say is that I love this club, I love Barcelona, I love the people here, it's incredible, and I give my all for this club.
"To be honest, I don't like these things when I see myself on TV, and I don't like my grandchildren seeing their grandfather like that, so maybe I have to change my behaviour."
Barca are enduring a challenging period with a spate of injuries to key players, including Robert Lewandowski, Raphinha, Ferran Torres and Dani Olmo.
Flick is left with limited options in attack and is likely to field a front three of Fermin Lopez on the left wing, Marcus Rashford as the central striker and 18-year-old Lamine Yamal on the right against Olympiakos.
"Marcus can be a number 9 or a number 11. When we think of him, we value the fact that he can play in more than one position," Flick said. "He has given us a lot of positives and, of course, he can play as a centre forward."
Barca are 16th in the Champions League table on three points from two games, while Olympiakos are 29th with one point.
The Catalan club have appealed against Flick's sending off, aiming to ensure he will be on the touchline on Saturday at Real Madrid, who second-placed Barcelona trail by two points in LaLiga, although the coach acknowledged it is a long-shot.
"I had nothing against the referee, I wasn't complaining at him but he saw it that way and I have to accept it," Flick said.
"Today the atmosphere was very good. I like to see this kind of spirit. We are a young team that needs this."