Whenever you activate your “vampire senses”, Vampyr’s take on the investigation mode which is so popular nowadays, most characters are highlighted with a visible circulatory system and an informational pop-up for this character’s basic info.
One thing visible in this information box for potential prey is a count of the number of hints you’ve revealed, versus how many in total are available for that character, alongside the gauge of how much XP you’ll gain for consuming that character. The more hints you reveal (through conversation with them or others, discovery of journals/letters/etc. in the world, a slightly finicky spying minigame, and so on), the more that gauge fills up; once you know everything about a character and they’re cured of all diseases, they’re worth max XP and you can eat them with impunity, right?
Well, no.
While the game tells you explicitly how many hints exist for each character, it gives you no indication whether that character is needed for another character’s hints, although at what is probably the half-way point through the game, these stories have so far been relatively self-contained.
The real mechanism to prevent you from prematurely murdering people you might want later on is “mesmerize level”. This is a ostensibly an indication of a character’s force of will, but in practice it’s just a number to tell you how soon you’re allowed to eat somebody. At the end of chapter 2 I gained the ability to mesmerize people of level 2 or below; I’ve seen people as high as level 5. That’s usually a strong indication of their importance to the plot.
Combat is still passable. I am convinced the post-death loading screen is deliberately slow to discourage players from taking on significantly higher-level enemies over and over in hopes of a lucky break.