Co-Optimus: How exactly does the co-op work? Does AI take over if the other player isn't there?
Relic: You can invite a friend to join you at any point in your campaign through Games for Windows Live. If they accept they are taken to your battle barge and given control of two of your squads. The battle barge is where you equip and upgrade your squads and choose which mission to accept. Your friend could be in the middle of a game when they get an invite and as soon as they accept they will be taken to the barge.
In a non-co-op singleplayer game, you have access to six squads and they all have different roles and capabilities. When you have decided upon which mission to attempt, you then choose three of the other five squads plus your force commander to deploy down to the planet. In Co-Op it is exact same principle, except for each player must pick which two they each want to take.
When you are deployed to the map, your co-op buddy is a slightly different variation in colour scheme than yourself so you can tell whose who. You are not able to control your opponents squads and vice versa, but to express dissatisfaction with their performance you can throw grenades at them, yell at them through voice chat (thanks to Games for Windows Live) or charge through their cover leaving them needlessly exposed.
If your opponent drops out or quits, you get control of his units.
Co-Optimus: Are there any special moves or features you can do in co-op that you couldn't do in single player? Perhaps a combined special attack or something?
Relic: There are none that are exclusive to co-op per se, but many of the abilities were designed with cooperative play in mind. For example your tactical marine sergeant Tarkus has an ability called Tactical Advance. When activated it makes him immune to suppressive fire and he will also take less ranged damage. This means that if an enemy heavy gunner is suppressing your units in area making them move very slowly and fire less often, Tarkus can break that suppression, move up and throw a grenade.
If you decided to invest points you earned from leveling up in Tactical Advance, it will then affect units in an area around Tarkus. Now when your buddy is suppressed or taking too much damage you can protect a less “beefy” unit like your scout marines. Another that Tarkus has is taunt ability which forces enemies to target him, perfect for saving your friend that decided to test the scouts melee prowess vs. an Ork with a giant axe.
Although many combination style tactics are available to the player when playing by themselves, I personally found that – unless you’ve got some elite micro, some of the more complex ones are much easier to execute when you have a buddy focusing on controlling your other two squads. The units have plenty of abilities and so by only controlling two of them it makes it much easier to focus on executing them correctly. On the higher difficulty levels, there is little room for error and so the more attention you can pay to a particular squad, the more likely you will eliminate your enemies before they can deal any significant damage.
Co-Optimus: If a co-op player fails - do both players fail?
Relic: If both you and your co-op buddy’s squads are knocked out, you fail the mission and are returned to the barge. However, as long as one of your squads remains (with the exception of the dreadnought) you can revive his units, similar to games like Gears of War.
Once you are back on the barge you can re-equip your units and try the mission again so long as it hasn’t expired, as some missions only exist for a few days.
Co-Optimus: One of the features that you've focused on is having players become invested in their hero and squad. Part of this investment comes from making choices about what missions to accept and seeing the affects those choices make both on your squad and the overarching story. Will having a friend along help out mean that you both can take on more missions, or is the second player tied to the same choices the first player makes?
Relic: If you invite someone to your campaign, they will be playing your campaign and it does not affect their own. If you are having a difficult time completing a mission or one of your friends has a spare thirty minutes you can invite him along. They can continue playing with you as long as you like but when they have to leave or if they happen to lose their connection, you can continue playing as normal.
You can’t take on any extra missions per se, but it may make the game a bit easier, earning you extra deployments, which do allow you to take more missions. This is obviously possible when playing alone, but in my experience it was easier to accomplish with a friend (especially on higher difficulty settings).
The other player is free to equip their chosen squad with what gear they want and upgrade their character through the leveling system as they see fit. My most recent co-op venture through the campaign saw me taking the melee specialist squads while my friend decided to use two ranged squads and upgraded and equipped them how he preferred.