This page in our guide to The Sinking City contains a detailed description of Brutal Honesty trophy/achievement. You get it by breaking the promise you gave to the guard near the fish storage.
You can get this trophy/achievement during Quid Pro Quo main case. At some point, Reed must examine EOD Fish Storage located in Shells district.
The fish storage is guarded by Daryl Grimes. Speak to him and then focus on looking for clues in the fish storage - check our walkthrough for this case.
Leave the fish storage and speak to Daryl again. He asks you not to tell Anna that he didn't do his job and that he let the intruder poison the fish. Agree to keep the secret.
Go back to the Fish Market and head to Anna. Choose the dialog option that breaks the promise - you tell the woman that Daryl failed and let the intruder enter the fish storage. The guard will be punished.