To finish this game 100% as fast as possible you will need to use save scumming. What that means is that you will need to make a copy of your game at a certain point so that way you can do alternate endings.

The save file is located in ...AppData/LocalLow/DigiTales Interactive/Lacuna

In theory you can 100% in 2 complete playthroughs, but I needed 3.

In addition to the 8 endings, which I will cover, there are also 3 mutually exclusive achievements.

Happy Boi vs Chonky Boi
Healthy Lifestyle vs Chain Smoker
Good Cop vs Bad Cop

Technically there is one more but it's after our save scum so you don't even need the 2nd playthrough for it.

Smoking
Healthy Lifestyle
You need to smoke zero cigarettes. The game will make you smoke sometimes and that doesn't count.

Chainsmoker
You need to smoke at least 10 cigarettes. There is a cigarette machine outside of Neil's apartment that will refill each time you're back. You can just stand at a smoking point such as his balcony and smoke them all.

You can check how maybe you have smoked in the Cell.

The achievement won't trigger until you are heading into the final scene.

The Lizard
Anytime when your objective is to sleep, interact with your lizard Horace FIRST. You do not get another chance after you have slept.

There are only 3 opportunities to do this so if you miss one, you will need to replay. The first one is early in Act 1 and the 2nd is late Act 1.

Happy Boi
Pet Horace 3 times

Chonky Boi
Feed Horace 3 times

Reunited at Last
If you want to get this achievement starting from Act 1, anytime there is an optional objective to talk to Catherine do it and be nice. I think it is best to visit with Laura too because it makes Cat happy.

Good Cop vs Bad Cop
This achievement is also in act 1. You need to be consistently nice and understanding when questioning people or just awful. People where it seems to have an impact

• bodyguards at Banny's house (and how you interact with Gary about them)
  
• bartender at Sakura hotel (don't turn her in or do)
  
• Ed and Rob at the docks (and whether you help Ed with a parking spot)
  
• Thompson and Sheila

Good cop achievement for me didn't pop until Act 2 for some reason but Bad cop popped once I got back to the train after threatening Thompson at the Lotus.

Groupie
When you are in Matuna Residences there is a band playing to the right of the stairs up to the witness. Stand in the smoking spot to listen to the band for about a minute. I did this on my smoking playthrough and smoked about 3 cigarettes here to get it.

Matching Description
Correctly answer Banny's Murderer sheet - Bald, cut arm, shaved

afk?
Perfect time to do this is while waiting at the Act 1 conclusion. Just don't touch anything. I found that this achievement didn't stack with Couch Potato.

Jorus Plaza
Finish Act 1

Starting in Act 2 your relationship with Zora matters for the endings! Be consistently helpful or not.

Good Zora

• agree to help
  
• give your own ID instead of Gary's (not sure how much this matters)
  
• say she was close to Boyt and offer to deliver a message
  
  
• give message to Boyt and later deliver his back to her
  
• share the details of the attack
  
• drink a beer
  Act 3 technically
  
• agree with her
  
• ask if she's consoling you
  
• talk about redemption and all human life having value

For bad Zora just do the opposite.

Crisis Averted
Submit the Broadcast sheet correctly - Moore

She's Got a Point
When talking to Zora at your apartment the 2nd time, she says something about all people being selfish and you have the choice to agree or talk about all human life having value.

Choose "I agree".

I did this on the bad Zora run and got the achievement but I think as long as you agree here you could get it even being mostly good.

Corion Church
Complete Act 2

A Little Justice Served
Submit the Liberators sheet correctly and then arrest him on the train - Dyson, Dvoria, Mining, Tyllanium

Is This the Right Thing
Answer the Data sheet correctly - Sector II, 1800, Dvoria - and tell the journalist

Or Is This The Right Thing
Answer the Data sheet correctly - Sector II, 1800, Dvoria - and don't tell the journalist

Rescue Mission
Complete Act 3

The endings hinge on 3 things, 1 of which is your relationship with Zora which starts in Act 2. You will need to be consistently nice and cooperative in one save file, and antagonistic in other.

Once you are 87% of the way through at the scene called "Hack" and it's time to submit the sheet called "data" it's time to exit and backup your save.

You can get 4 endings with each playthrough:

1. Submit the correct data sheet, threaten Saito
2. Submit the wrong data sheet, threaten Saito
3. Submit the correct data sheet, tell the CDI
4. Submit the wrong data sheet, tell the CDI

Bring Her Home
Save Laura

Since you are going for all endings, most of them save her. You'll get it easy.

Will It Have Mattered
Finish the game once

Enter the Multiverse
Get all 8 endings (you don't have to watch the credits, just wait for the counter to update and Alt+F4)

Get all sheets correct. Prologue doesn't count!

The Bullet - Sakura Hotel
Diaz's Escape - Bartender
Banny's Murderer - Bald, cut arm, shaved
The Broadcast - Moore
Moore's Motive - His sick wife
The Liberators - Dyson, Dvoria, Mining, Tyllanium
The Data - Sector II, 1800, Dvoria

I needed a 3rd playthough for this but in theory you could get it on your second playthrough once you have a sense of where you're going.

You need to take less than 5200 steps to get this. You can see your steps in your Cell.

Some tips:

• Do it on a run where you fail the sheets (in my successful steps run I failed every sheet - for science)
  - failing the Diaz escape skips the docks (not sure if this screws up Good/Bad cop)
  - failing The Broadcast skips the Restaurant with Cox's wife
  
• Ignore optional objectives
  
• Threaten Thompson to avoid having to talk to Sheila
  
• Starting in Act 2 you can take an actual shortcut by walking right from the train towards your house and walking up the path that was previously blocked
  
• Try and reason with Cox by talking about his wife and you'll get shot and you don't have to walk out of the building
  
• If you're doing it on a fresh save, pick the ending where you submit the incorrect data sheet but tell the CDI - you don't have to walk as far as any other ending

At any point sit on a couch or chair and just afk for 20 minutes.

Funnily enough, the afk achievement didn't trigger at the same time. I assume since maybe you need to be standing for that one. I have a note for a good time in the Act 1 section.