Rescue Sinthia: If you spend the night at the Crash House, Marcelles will ask you to Save Sinthia from a crazed raider in the room next to yours. Once completed, Sinthia will now be available for "services" (including, but not limited to, the Tell me about feature).
Interestingly, when asking Sinthia to "tell me about" various words familiar to her profession, she feigns ignorance.
If the Vault Dweller used her "service" before, once in a while if the player return to Junktown and walk past Crash-House Inn, there will be a text that says you "feel a sudden urge to visit Sinthia again".
Playing as a female, Sinthia will not be available for "service", but if you have a higher luck, she will allow it once when talking to her about her boss directly after rescuing her. If you fail to persuade her, she will instead quote both of you being women.