This achievement can be missed. There are discussions which can only appear in ACT 2 or ACT 3, and some are trickier.

This achievement is only linked with "Let's talk about something.". You don't have to see all their other dialogs, and you will unlock this achievement during ACT 4, not after.

I'll keep spoilers to a minimum and put in tags where necessary, and I will say no information post ACT 4, but this guide is mostly recommended for a second run. Enjoy the game before, to know if you want all the achievements. But if you really want all of them, yes, read this guide to not miss achievement during your first run.

Every time you speak to Loop, each accessible events have (more or less) the same probability, so if you want to see specific temporary events, loop back to see it before you move forward in the story or in secondary triggers.

There are 23 conversations which unlock the last one, but you have to see 21 of them, or 23 of them. It's related to whether or not you took the dagger. More information later.

You must have the dagger for 2 of their dialogs, so I highly recommend you to take it and to keep it. If you really don't want it, you can do the 21 other events to obtain this achievement. But be aware that the achievement 1000 ways to die requires that you use it.

The garden of the 2nd Floor is your worst enemy. There are 3 conversations about it, but the first two can be easily missed.

A convo about the Silver Coin can only appear if you don't see "The Incident". This event can be trigger starting from the ACT 3, so don't use your Silver Coin in you inventory during ACT 3 and 4 before you see this event. It's random but you can trigger "The Incident" anytime by using the coin during these ACT if you are not next to Loop.

A convo about the King appears after you defeat him, but disappear after a certain moment of the story (when you speak to him about Time Craft). So do it quickly.

Touch your party in the garden of the 2nd floor 2 or 3 times (NOT MORE)

The garden is a room in the west of the 2nd Floor, easy to find. To "touch" you party : Walk around the table, past the NPC frozen in time, and go next to the plants. Then retrace your steps until you're in front of the entrance of the room, where an interaction should occur.
Do this 2 or 3 times and not more (by looping back), and go talk to Loop (saying "Let's talk about something."). You need to loop back and come back to talk to them until you've had that conversation so you don't miss it and you can do the next one.

Touch your party in the garden of the 2nd floor 4 to 8 times (NOT MORE)

You don't have to do the previous event to have this, you just must have 4 to 8 "touchs", so make sure you've seen the previous dialog before.
Same here, return to the garden to "touch" your party members. You can loop back to repeat this. Interactions will be different, but you have to return to speak about it with Loop when you've done you 4th interaction. (the 8th at the maximum but less is better, just in case)

The last garden dialog can't be missed so feel free to poke your party members now. (But the last garden conversation seems to be only ACT 3 or later.)

Do NOT see "The Incident".
WARNING : This chat has 1 in 23 chance of triggering, and if not triggered, the game can act like you don't have anything to talk about (even if you have not seen the incident). So keep trying until you see it. (Thanks to Amla for noticing this!)

Do not use the Silver Coin during ACT 3 and 4 before you see this conversation to be sure. I recommand you to trigger this during ACT 2, so you will not think about this later (because it's funny to use the Silver Coin).
"The Incident" is an event which have a low chance to happen during ACT 3 if you use the Silver Coin (and that you are not next to Loop). During ACT 4, there is a lot of chance for it to happen. You just have to know you can't take any risks.

No other condition, do it during ACT 2.

No other condition, do it during ACT 2.

Use the dagger 10 times or more.

The event where you can choose to take the dagger is triggered during ACT 3 or 4, when you talk to Loop after you have used 20 tears or more to loop. They will speak about tears, and after this, you have to choose to think of an other way to die and choose to take the dagger. (If you want to have all the achievement in one run, you have to take it for 1000 ways to die)

Choose to keep the dagger when Loop ask you about it.

Loop will speak to you in an other loop after the event where you obtain the dagger. Make sure to keep the dagger if you want to see this conversation.

I think you can "give it back" and do the 21 other dialogs, it should be ok for this achievement. If you use it one time, you can have 1000 ways to die (now or later) but you don't have to keep the dagger even if you want all the achievement in this run.

You can see this dialog after you've beaten the King for the first time.
Note : It seems it can also be triggered during the ACT 2 after you lose against the King.
BUT ! You have to see this before you speak to him about Time Craft

5 loops or more (You will do these 5 loops, but you have to see this before ACT 4)
Depending of your moment in the game Siffrin can say something else than "friends".

No other condition, do this before ACT 4.

No other condition, do it before ACT 4.

No other condition, do it before ACT 4.
Same here, it can be other word than "friends".

Maybe after the related Isabeau's subquest, but you will see this for sure.

No other condition.

No other condition.

No other condition.

No other condition.

No other condition.

No other condition.

Touch your party in the garden of the 2nd floor more than 12 times.

Talk to Loop more than 20 times.

After the dagger conversation, which is unlocked by looping 20 times with tears (so during ACT 3 or 4).
Even if you don't take the dagger.

20 loops or more.
The first sentence of Siffrin is different if you have it during ACT 2, but it's considered to be the same dialog.

When you've seen all this 23 conversations, or maybe 21 of them (if you don't take the dagger), good job! You've done it!

You just have to loop back and return to Loop to have your last conversation and to obtain the achievement!

I've made a guide for this achievement specifically because it's too hard to have it without doing a new game, and it's still hard if you do a new game!

If you keep a save during the end of ACT 4, you can obtain almost all the achievement. There are just two which can't be obtainable during or after the last part of ACT 4 :
-This achievement: "Behind the scenes"
-"...You are loved" (the conditions are simple but you can't to this at the end of ACT 4)

So I recommand to keep a save during the ACT 4 (anywhere, like in the beginning) and a save at the end of ACT 4 (Like, the moment when you can go to ACT 5. Just save everytime and make an other save when it's written "ACT 5", it's not hard to do this.)

Don't forget to use the Silver Coin after you've seen the dialog with Loop about it. See "The Incident" and obtain "Here is some change" by using your Silver Coin anywhere but not next to Loop, and then use it next to Loop to obtain "What was that about... ?". There are two achievement related directly to this coin, and you can't miss it during ACT 4. When you have these, you can do ACT 5 with no trouble (believe me). But if you do all the secondary quest during ACT 4, you will have more dialogs on the epilogue. And keep a save during the epilogue (other than the ones during ACT 4, of course!), you can return to the village during it, and if you do the events of Here is some change and What was that about... ? during this run, you can obtain a new event and achievement at the Favor Tree.

OP achievement is time-consuming but easy, you just have to xp your characters to the level 99 for Siffrin and 75 for the other, on any loop in the same time before the boss of the 3rd floor. (Floors boss are good to xp your characters (and you can farm the 3rd to xp Siffrin), but be max level after beating this boss can not give you the achievement. (Don't forget to use the Memory which give more xp to your members (story related))

The achievement with the "three ghosts" can be hard to do, but you can save and load before going to the hallway where each appears in each floors.
1st: Left hallway past the locked door.
2nd: Right hallway from center room.
3rd: In the hallway after the bathroom, before the keyknife.

Last tip! 3 Memories can be hard to find! The Memories of Pillars, the Memories of Barrils and the Memories of Fishing! But they are aptly named, so it's ok!

That's all for me, thanks for reading! And thanks Adrienne and all the people who have worked on this fabulous game!