The Chelsea game demonstrated the problem of defending a winning position when playing at home. The dilemna as to whether to stick or twist. If it had been 0-0 in Paris, Chelsea would have gone at PSG in the first half and probably been 2-0 at HT. But Mourinho's inate caution meant that they were prepared to take 0-0. That negativity kept PSG in the tie, even when Chelsea scored.
Barca aren't Chelsea and will undoubtedly come at us from the start, hoping to put the tie to bed in the first 30 mins. But if we're level at half time the doubts will start to creep in. Do they keep going for it or settle for what they've got? Take their foot off the gas, perhaps with one eye on El Clasico. That's when they will become vulnerable.