There are 5 achievements tied to completing the 5 Acts of the game. Act 2 is roughly 50% of the total game.

There are no hidden achievements in Act 1

First Trust - During your first cutscene at the apartment with Renee, make choices showing that you trust Renee. And choose that you trust her, when making tea.
Not Born Yesterday - During your first cutscene at the apartment with Renee, make choices showing that you doubt / are suspicious of Renee. Choose that you do not trust her when making tea. (credit to Pride Charm)

Enabler - In Gastown, before the fake truck arrives, give cigarettes to kids out front of the Den (right from the start), and don't tell them that cigs are bad.

Sins of our Fathers - In the Minister's house, peek at what the grown ups are doing at the dinner table, then tell Jebediah that his parents are eating flesh of other people. (credit to Pride Charm)

Trout Talker - In Gastown, at the Liquor Store, talk to the Tuna 25-30 times. It will eventually explode.

Tonic for the Soul - In Gastown, at the Liquor Store, reassure Louis in your dialogue choices.
Bad Medicine - In Gastown, at the Liquor Store, be mean to Louis in your dialogue choices.

Badge of Honour - In the West End, when speaking alone with Henry, let him hold your detective badge.

NPCs needed: Renee, Ingrid, and Clarissa in Act 1 at The Bite. Superintendent, Beverly, and Samson in Act 2 at West End.
Howard Lotor - In Act 1 and Act 2, tell the truth about who you are (Howard Lotor) during every conversation.
Wallace Brotor - In Act 1 and Act 2, lie about who you are (Wallace Brotor) during every conversation.
Brollace Howtor - Swap back and forth between being Howard Lotor and Wallace Brotor when speaking to people in Act 1 and Act 2. (NOTE: I have alternated between the 2 names, but haven't gotten this to trigger)

NOTE: One or more of these may get bugged, and you will have to delete your achievements.sav file + disable Steam cloud in order to trigger them. You should only delete achievements.sav after earning the book and sniff achievements (if you want to earn them).

Snoopy Snooper - At Rose's house, on her computer, look at every file.

Keep the Hope Alive - After leaving Rose's room, but before leaving Rose/Florence's house, speak with Florence again and reassure her with your dialogue choices.

There are no achievements that trigger in Act 3 on a 1st playthrough.

Me Time - Get firewood without talking to anyone in the first open-world section. In the second, continue not talking to anyone... sit at the water edge, sit on the edge and become the forest, and then return to your tent and take a nap.

Geezer Pleaser - In Act 2, pet the geezer. In Act 2, be nice to the delivery geezer and tip them. In Act 4, be respectful to the geezer under the Bridge, and then pet the geezer.

New Habits Die Easy - Rat out Joshua to Roger that he's smoking in the woods.

These can be done across multiple playthroughs:
Affirming Flame - Be nice to Eddie and Belle. With Eddie, make choices to convince him to talk with you without threatening violence or using Acid. When breaking out of the container, put the wet bandage over your face and then feign being unconscious. Then be nice to Belle when you leave. (credit to Lanze and Nuts). You can also get this achievement by being nice to Eddie in Act 3.
Cruel to be Kind - After you tie up Eddie, and he answers your 3 questions about items in his lab, you have a conversation with him. Be mean, and threaten to pour acid on him. With Belle, using the tray, feather, fishing bone, butter, and bandage: wet and then stuff the bandage in the vent, pick the lock, and hit Belle with the tray.

There are no achievements in this act

NOTE: If you miss any, the achievement should trigger upon getting the final item you are missing in a subsequent playthrough. You do NOT need to get all within 1 playthrough.

Blurb Herder - Read every bit about every book in the game, from all shelves and NPCs. The achievement will trigger when trapped in the container, after the cutscene where you're experimented on, and you read the front cover, middle, and back cover of the book left for you.

• Howard's bathroom at the start of Act 1
  
• Book Store in Gastown in Act 2
  
• Bookshelves of the Minister's Office in Act 2
  
• At the apartment building in West End, 5th floor, in Act 2
  
• The book about flowers in Florence's apartment, in Act 2
  
• Belle's book in the container in Act 4

Smell-o-Vision - Sniff everything that can be sniffed. Will award from the bandage in the container in Act 4 if you did not miss any.

• Jeremy's dead body in Act 1
  
• Flesh in the crate at the liquor store in Act 2
  
• Minister's desk in Act 2
  
• Flowers on the roof in the Science City garden in Act 3
  
• The air and your tumor at Larry's Mirror in Act 4
  
• The bandage on your tumor in the container in Act 4

True Connection - You need to talk to do lots of things involving Renee, either directly or indirectly. Here is a comprehensive list of things (some may be unnecessary, i'm not sure, but better to be safe!) (credit to the Backbone discord: P3nguin, MrFalcon, Smoke, J2B9, and others)

• Be honest in first meeting in Act 1 (might not be needed)
  
• Be honest / loving throughout talks in Act 2
  
• Ask the book store person about Renee's book in Act 2
  
• Call Renee in Gastown in Act 2 to say you're thinking of her
  
• Answer everything accurately when analyzing Minster documents in Act 2 (probably not needed!)
  
• Call Renee in West End twice in Act 2. After speaking with Henry and after speaking with Samson.
  
• Don't reveal Renee to Samson in West End in Act 2
  
• Give Clarissa the "Renee" mug (might not be needed), but then deny working with Renee. Tell her Ellen instead. In Act 2
  
• Have Larry check on Renee / ask about Renee in Act 4 (might not be needed)
  
• Think about Renee when speaking with the Artifact in Act 4
  
• Write that you love her in the letter to Renee in Act 4. To get that line, pick the 2nd option for your 1st line of the letter

Hope everyone enjoys the guide. Please give it a thumbs up and stuff. -Tass