343 Industries has described on their Halo Waypoint blog more gameplay details that teammates can do in Halo 5: Guardians.
343 Industries described in previous Halo games, the single player campaigns were mainly designed for only one player. They mentioned a friend joining in on those games was an extra bonus.
For Halo 5: Guardians, the campaign has been built for co-op gameplay in mind. They say you can still experience the game as a solo player, but playing it during co-op will add “a new level of depth and richness“.
Solo and Co-operative modes will have their own save files and online servers. They also said players can easily drop in and drop out of gameplay without any interruptions. The AI will simply take over if one of your online mates needs a break.
With co-op gameplay in mind, 343 Industries have made some alterations to the mechanics of the game as well. They have added a new “Revive mechanic“. When a mate goes down, you can revive them during the “downed” state. You have to be quick on reviving them because if you’re too late, your mate will die. If you are down, you can easily call out for help.
If you die, you can only respawn when your mates reach a safe area. If every player dies, you will revert back to the previous checkpoint. To put it bluntly, try not to die all of the time.
Finally, 343 Industries explained that they have designed the levels to make them bigger to allow for co-op gameplay. They have made different pathways so teammates are encouraged to spread out and explore. They also mentioned there will be more enemies because of the co-op mechanic too.
343 Industries ended the post by saying they’re sorry for removing split-screen gameplay in Halo 5: Guardians. They explained they needed to do this to deliver “a true next-gen experience“. The improved AI, bigger levels and good graphics are the reasons why they had to drop split-screen.