sometimes you can see (or hear) another player in the distance or left or right from you fighting. there are often 2 ways in a mission, so in one way it is you, in the other another player doing the same mission.
sometimes you have missions like the 7th which you can play as V or Nero, when you play V for example, a Nero player can randomly join your game for some co op action, i´ve had this on my second playthrough. sometimes you don´t see anyone because they are fighting somewhere in the same level but not at the place you are at the moment.
