Probably the big difference is using the thumb differently. In flamenco you (can and usually) press the thumb with the entire hand and do not move the thumb seperately, where as in classical you usually don't see a motion in the hand, but only in the thumb. By moving the hand you can deliver force, for the explosive sound. I really had to get used to that as a former classical player. Also the tremolo might be difficult, I'm used to a flamenco tremolo now which is different than classical. I find it hard to switch between these.
2013 Perez Luthier India 'Abeto'