Brad, Alex, Julia, Conrad, and Fliss are the characters you need to keep alive for this achievement.
This guide is spoilered as it goes through each chapter where you can possibly get a character killed, and how to avoid this. In general, you don't want to fail QTEs - the timer for these can be turned off (single player only) in the Accessibility settings.
I'd recommend going for this achievement *not* on your first playthrough, so you can familiarise yourself with the sections and whatnot.
THEATRICAL CUT GUIDE:*** Spoiler - click to reveal ***
Arguably the easiest ending to go for this achievement in Theatrical Cut is by saving the distributor cap, with the Matters of the Heart ending chapter (requires Alex to be by the radio near the end) - so this is what the guide will be aiming for.
See the ALTERNATE ENDINGS section at the bottom if you'd prefer to go for a different ending.
KEY PARTS IN EACH CHAPTER:
Dive (as Julia)
- Do not surface too quickly at the end of the dive (Cautious option).
Intrusion (as Conrad)
- Do not escape on the speedboat (fail the Keep Calm, QTEs, or grab the knife instead of escape).
Finding Friends (as Brad)
- Find the gas mask. This is found by opening the locker twice in the locker room (first time is a jump scare, second time the mask is there).
Ritual (as Fliss)
- Make sure you go outside in good time (through the door at the end of the ballroom backstage, after you've opened the curtains, this will stop you hallucinating and possibly attacking Brad). This should be done fairly quickly, i.e. right away or within a minute or so, as this chapter is timed.
Pressure (as Fliss)
- Go down the hole as Fliss (Confident option).
- Insist Brad smashes the HINGE.
Glamor Girl (as Conrad)
- Do not attack the glamour girl at all in this chapter.
- Do not fail the QTEs or the Keep Calm.
- CONFRONT the glamour girl at the end of this chapter instead of jumping.
Revenge (as Julia)
- Do not fail the QTEs.
- Choose to JUMP (instead of run).
Distress Signal (as Alex)
- Make sure Brad goes down the hole at the end of this chapter (Encouraging option).
Depths (as Brad)
- Take the rebreather (Assertive option).
Junior (as Brad)
- Do not fail the first Keep Calm.
- Then, choose to run, and do not fail the QTE(s).
- Reassure Junior you did not breathe in the mist (Curious -> Worried -> Desperate options).
Matters of the Heart (as Alex)
- Do not fail the QTEs when jumping across the beam.
- Do not fail the QTEs when the rats attack you while grabbing the distributor cap.
- Do not stab the monster or the rat that comes out of your chest.
--
ALTERNATE ENDINGS:
If you don't want the distributor cap ending, here are some differences you can make to the guide above for an alternate route:
Coastguard ending
- Intrusion: Get Conrad to escape on the speedboat (do not fail the Keep Calm / choose ESCAPE / do not fail QTEs).
- Matters of the Heart: Get the distributor cap destroyed by failing the rat attack QTEs.
Military ending
- Distress Signal: Do not inspect the Ourang Medan log book in the radio building. Tell the military your coordinates on the radio.
- Matters of the Heart: Get the distributor cap destroyed by failing the rat attack QTEs.
CURATOR'S CUT GUIDE:*** Spoiler - click to reveal ***
We will be going for the coastguard ending (Conrad escapes on speedboat, distributor cap is destroyed) in Curator's Cut mode. It's definitely harder to get the achievement in this mode, as you will not have control over certain characters in critical moments.
KEY PARTS IN EACH CHAPTER:
Dive (as Julia)
- Do not go into the hole without your gear whilst exploring the wreck. Julia will end up surfacing too early, but this should also cause her to listen to Alex and thus not drink a beer later on (preventing the bends death).
Intrusion (as Alex)
- Encourage Conrad to escape on the speedboat (Enthusiastic option).
- Knock open the window at the right time (when Conrad has counted to six).
Ritual (as Fliss)
- Make sure you go outside in good time (through the door at the end of the ballroom backstage, after you've opened the curtains, this will stop you hallucinating and possibly attacking Brad). This should be done fairly quickly, i.e. right away or within a minute or so, as this chapter is timed.
Finding Friends (as Brad)
- Find the gas mask. This is found by opening the locker twice in the locker room (first time is a jump scare, second time the mask is there).
Pressure (as Brad)
- Inspect the broken shovel.
- Let Fliss go down the hole.
- Listen to Fliss and smash the HINGE.
Revenge (as Alex)
- Do not fail the QTEs.
- Choose to JUMP (instead of run).
Depths (as Julia/Fliss)
- Take the rebreather (Assertive option).
Junior (as Julia/Fliss)
- Do not fail the first Keep Calm.
- Listen to Brad and stay hidden.
- Do not fail any QTEs.
- Reassure Junior you did not breathe in the mist (Reassuring option).
Matters of the Heart (as Julia/Fliss)
- Go straight to open the door in the room with the collectibles. Doing this quickly enough will prevent Alex from getting the distributor cap.
- Survive Alex (do not fail QTE, then Commanding -> Encouraging -> Desperate options)
If you've followed this all correctly, all your characters should be alive and the achievement should unlock post-credits. Please let me know if you come across any edge cases that I've missed out.
Achievement complete!