In order to get the Fibber achievement, you will need to tell 10 unique lies. I believe those lies need to be on one save file, and I don't think bluffs count.

This guide is not recommended for anyone who hasn't already completed the game, as it contains spoilers to various quests; however, those spoilers will be kept light. NPC prompts will be either hidden by spoiler text, or truncated. Where necessary, the conditions needed to start conversations will be kept vague enough to avoid spoiling solutions to quests; however, some of these lies only crop up when solving a quest, so they are unavoidable. This guide also does not include the several lies you can tell to end-game characters.

There are three lies listed which you can get permanently locked out of being able to say, based upon the decisions you make in a save file. If you want to tell them, you may need to start a new save file; however, doing so is unnecessary for getting the Fibber achievement, as there are ten unrestricted lies (lies which you can't get locked out of) listed in this guide.

Lies are categorized by which character you tell them to, and those characters are ordered alphabetically.

As always, this guide is dedicated to every jackass who thinks that a youtube link is an acceptable substitute for a text guide.

The dialog trees that lead to a lie are presented like this

Conditions: Any conditions that need to be met in order for the correct prompts and responses to appear. Unless noted otherwise, assume the Golden Rule needs to not have been broken in the current run, and the conversation probably needs to happen pre-election. If a character refuses to speak to you, it may be because you chose a dialog option that angered them, in which case you will need to try again next run.

These lies have conditions on them that make it so you can be prevented from telling them again on later runs. These lies are sorted by the approximate order in which you will encounter these characters.

Conditions: First conversation on a save

Conditions: Have not confronted Sentius about his secret on any run. Have spoken to Sentius at least once during this run. Note that this conversation can also be held during your first conversation with him on a save; however, it takes less steps to get it during any other conversation.

Conditions: Obtained wine, and spoken to Malleolus about Quinctius, and have not given Claudia wine on any previous run.

Conditions: None. Note that unless you have personally prevented Fabia from making a fatal mistake, this line of conversation will lead to the Golden Rule being broken.

Conditions: Prevented Fabia from making a fatal mistake. Note that this line of conversation is mutually exclusive with the previous one, so they will need to be done on separate runs. It is possible to get this lie in the previous line of conversation, but for the sake of simplicity I've kept it separate.

Conditions: Personally prevented Fabia from making a fatal mistake, and dealt with the Assassin without breaking the Golden Rule

Conditions: You have talked to Galerius at least once during the current run, and he doesn't need to talk to you about running errands.

Conditions: Told Galerius to urgently complete a task for you, and Galerius completed that task.

Conditions: Obtained silphium resin, and Iulia is not dead, and Iulia has not been rescued in this run.

Conditions: Seen the evidence proving Malleolus' true identity.

Conditions: Have spoken to Ulpius at least once on a previous run. Saved Iulia this run. Currently holding at least 2000 denarii. Have not blackmailed Malleolus. Note that the first two prompts are spoken by Octavia.

I hope this guide helped. Let me know if you've found any other lies in the game, and I'll be happy to add them and credit you for them. I just ask that you try to avoid outright spoilers in the comments.