The first thing you might notice is that this is a partner-based game, and you will be switching between partners throughout the campaign. In the Claire-Moira campaign, Claire is your fighter, and Moira can use her flashlight to stun enemies, find resources (shiny objects), and use the crowbar acquired in Episode One to perform melee attacks. In the Barry-Natalia campaign, Barry uses weapons, and Natalia can see enemies through walls (and when they are invisible), can see enemy weak spots, and can crawl into small holes between walls. There are also special crates and chests in each level that only Moira or Natalia can open. At certain points you will need to do partner actions (boosts, open doors, pull levers, etc.).
One useful tip that they don't tell you until later episodes is that you can tell your partner to wait or follow (hold down and hit to tell your partner to follow or to tell them to wait), which can be helpful if you need them to stay in one spot or get out of your way/stop stealing your kills/stop doing things that could mess up an achievement.
This tip of telling your partner to wait is incredibly useful in Invisible Mode/harder difficulties. Moira and Natalia's health regenerates, while Barry and Claire's don't. This means you can leave your main partner behind in area full of monsters (as long as they haven't noticed them yet) and run to the exit to the next area with the support character. Before leaving an area in this manner, you have to tell your partner to follow you again or the game won't let you leave without them.
Purchase the ability to do follow-up attacks (called "Follow-Up") as early as possible as this is required for several medals. Also, purchase the abilities Laying Bricks and Batter Up to increase the power of the brick and the crowbar to make brick hits and crowbar kills easier.
Items you get in a main campaign carry over to the next episode you play, including replaying episodes out of order. This means that if you have some explosive bottles you didn't use in an episode and a medal in another episode wants you to kill enemies with sub-weapons, the bottles will be available while playing said episode. However, items don't carry over in the Countdown and Invisible modes, so feel free to use everything you have.
A small tip for Invisible and Countdown modes is that, while running towards a door, leaving the button pushed will open the door automatically, which is useful if a chasing enemy is close to you and about to attack. If the door needs a key, leaving the button pushed won't work.
Important: Don't buy the ability "Unholstered" until after you complete all medals. If you buy it before and you have a medal to do with Natalia or Moira, AI Claire/Barry can mess it up by shooting at your enemy. Some people have also said that the AI shooting voids the achievement of the knife-only run of Episode 1, but this can be avoided by defeating all the enemies in an area before switching to your support partner.
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