Mr Light, when following those two dudes just move between different hiding spots (benches, groups of people, carts of hay, tall grass...). I don't know which eavesdropping mission you're referring to (there are a few) but that should help. If it doesn't, just look up the mission on Google.
When it comes to combat, you can generally mash the strike button (X on PS3). However you can't actually mash the block and counter buttons. Though AC games are pretty easy when it comes to timing (when compared to Arkham Asylum/City for instance). Hell, time even slows down a bit when you block a "badass" enemy.