When Ashley says she's going to search for batteries, go to the siblings' bedroom and interact with the alarm clock. After that, do not talk to Andrew, but interact with the TV instead to keep the batteries. They're necessary to unlock the Vision Room in Episode 3.
At the time of writing, there is no achievement for collecting all stars and visions for the Vision Room, but that might change in the future, so they're included in this guide as well.

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When the siblings enter the neighbor's apartment, interact with the jacket in the top left corner and choose "Split with Andrew."

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When Andrew starts to chop the neighbor up, he tells Ashley to go get plastic bags. Instead of leaving, talk to Andrew and choose "Stay and watch."

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After Ashley cleans up the blood in the neighbor's apartment, the warden is about to enter it. Go to the balcony to get the achievement.

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Complete Episode 1 and you get the achievement.

At the beginning of the episode, interact with the soft drink ad on the right in front of the motel. If you missed it, you can still interact with it right after the siblings flee the motel room after Ashley's dream.
After the siblings flee the motel room and enter the street, there is a cutscene where a cultist walks past them. After that, instead of going to the cultist place, go back to the motel, interact with the vending machine next to the stairs and choose "Fine..." Then talk to Ashley to get the star.

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After Andrew is done in the cultist place and returns to Ashley, there is a cutscene where the hitman enters your motel room. Instead of entering the motel room:
- Go back to the cultist place
- Enter the door on the right that leads to the kitchen
- Interact with the elevator door in the top area
- Go back to the stairs to trigger a cutscene
- After the cutscene, interact with one of the red eyes on the ground
- Light up all nine candles (they're marked in the first screenshot)
- Turn on the music on the left side of the counter in the top area (look at Andrew's location in the second screenshot)
- After the cutscene, leave the cultist place

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After the hitman enters your motel room, enter it as well and interact with the closet in the top area (where you picked up your gun earlier).

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When you lure the hitman into a trap, you need to choose between "Shoot." and "SHOOT!!!!!!".
You have to choose "Shoot.", or otherwise, the achievements "Prophecy Fulfiller", "Splat!" & "Shots and Such" become unattainable in this playthrough.

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This happens during Ashley's vision when she falls asleep while they're in the hitman's car.

The circles show the locations of the flowers and the present you need to pick up.

The numbers show which paths you need to take and the correct order to go. The first area can be completed by choosing other paths, but you need to make sure that you have one flower left in your inventory before entering the second area. You also get Star #4 for picking the present up.

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In the dream that Andrew has while he sleeps on the sofa, interact with the 302 Lady's corpse to get the vision that shows how hekilled her.

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While you prepare the ritual to sacrifice your parents, you need to make choices that determine whether you get the "Burial" or the "Decay" route.

For the "Burial" route (at the time of writing unreleased), choose "No. But trust the trinket." and then "Decline".

For the "Decay" route, choose "No. But trust the trinket." and then "Reflect". You also get Vision #2 for this choice, which shows the joke endings. The Decay route has one exclusive achievement in Episode 2.

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This achievement is exclusive to the Decay route, so while you prepare the ritual to sacrifice your parents, you need to choose "No. But trust the trinket." / "Reflect".

Later in Ashley's vision you will get to an area with a door with a black lock in the top area. Pick up the flowers marked with green circles, follow the way marked with numbers and pick up the black key marked with the yellow circle in the top left corner. After you pick it up, another black key appears next to the door. Pick it up, too.

In the following area, enter the door at the top that is marked with the circle.

Talk to the Demon in the top left area that is marked with the circle.

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At the end of Ashley's vision in the Decay route, you need to choose between "BANG BANG, MOTHERF.!!!" and "Nah, it's okay".

Choose "Nah, it's okay" for Decay route 3 #1, or "BANG BANG, MOTHERF.!!!" for Decay route 3 #2.

You have to play all sub-routes to get all achievements.

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Complete Episode 2 and you get the achievement.

These achievements / choices / collectibles happen before the sub-routes split.

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When Grandma tells you to pick berries, pick up the trowel in the garden, enter the forest, dig out the capsule in the bottom right corner, and choose "TIME TO SNOOP!!".

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In the gas station, when the two officers arrive, interact with the magazines. After one of them says "I'm good. Thanks", immediately go to the right to avoid getting detected. Then interact with the bathroom door, talk to the cashier, and leave the gas station.

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After you leave the gas station, in the third forest area, there are two exits. You have to enter the one in the top left, or otherwise, the achievement "Shots and Such" becomes unattainable in this playthrough.
The arrows show the directions in which you need to move the rocks. When the way is clear, enter the exit and pick up the ring. You also get Star #5 for this.

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After Ashley wakes Andrew up from his vision while they're in the car, you need to choose between "Andy" and "Andrew". Choose "Andy". This leads to a shorter route. Complete it and you get the achievement. After that, the game lets you return to the choice and this time, choose "Andrew" to get back on the main route.

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If you have chosen "Nah, it's okay" in Episode 2, interact with the phone in the camping area parking lot and choose "Test her."

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After the parents in the camping area get sacrificed, you track the boy down to the parking lot and he locks himself inside the car.
Pick up the rock on the right, interact with the driver's door twice and then interact with the boy.

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After you've dealt with the boy and return to Ashley, the Demon summons a portal and a timer starts. Let the timer run out and you get the achievement.

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After you enter the aforementioned portal, the split between the sub-routes begins. If you have chosen "Nah, it's okay" in Episode 2, continue with section "Decay route 3 #1". If you have chosen "BANG BANG, MOTHERF.!!!", continue with section "Decay route 3 #2" instead.

After you're done in the tower and enter the portal, you get to the Demon's home. Interact with the RC car on the right to unlock the Vision Room.

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After Ashley kicks Andrew out, the Demon sends Andrew on a mission where he needs to solve several puzzles. Pick up all flowers you can find. Eventually, you end up in an area with a large blood circle in the top. On the left, there is a small island with a mug on it. Pick it up to get the vision. In this vision, Renee suspects that her children have an intimate relationship.

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In the room with the murder scene, pick up all seven documents, put them in the folder, pick up the ringing phone and always choose "FALSE" in the questionnaire.

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In the room with the wedding cake, take the key from the pink bunny, pick up the green bunny and put it next to the pink bunny. Interact with the pink bunny and choose "Absolutely!" Then interact with the female figure on the wedding cake and choose "Choker".

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At the part where Andrew needs to sign Ashley's suspension notice, interact with her homework on the sofa and choose "Fine..." The solutions are:
5 + 2 = 7
4 - 1 = 3
3 x 3 = 9
1 + 7 = 8
9 - 8 = 1
2 x 3 = 6

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In the dressing room adjacent to the theatre stage, talk to all Ashleys and always choose the middle option until all are dead.

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In the area that looks like a broken up version of the apartment, interact with the phone in the bottom left to get the vision. In this vision, Julia talks about the threatening phone calls.

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You get this achievement for completing this route.

When Andrew tracks Ashley down to the large blood circle, she runs around. You have to catch her to get the achievement. This is a timed event, so you need to do it as fast as possible.

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At the aforementioned large blood circle, instead of killing Ashley, wait until a cutscene plays. You automatically get Vision #5 after that cutscene. In this Vision, the parents realize that their children killed Nina.

Much later, after Andrew wakes up from making love to Ashley, pick up the gun and trinket on the ground. Then go to the kitchen and Andrew steps on the ring you picked up in the forest. Save at the choice between "Whatever. Drop it" and "Hold onto it". Both choices grant their own achievements when you complete their sub-routes.

Thanks to user CoolStory, who explained how to get Unknown Summon in this thread: https://steamproxy.net/app/2378900/discussions/0/6993585599475841022/

Thanks to user tigerboy61, who explained the faster way to get Vision Watcher.

Thanks to user Redwing134 for the information that the area for the Vision Watcher achievement has a missable key.

Thanks to user Zorth, who explained how to get Undetected in this thread:
https://steamproxy.net/app/2378900/discussions/0/594019268905935752/#c594019268905944874