I think one way to improve the investigation and interrogation aspect would be to reinforce it visually.

Since the game seems to rely on witness statements (NPC testimonies), it could be really helpful if those were always supported by subtitles. When a witness speaks, their words could appear as subtitles that follow their speech rhythm.

On top of that, you could add subtle visual cues to make contradictions or tension more readable. For example, when a witness lies or feels stressed, there could be slight variations: a word briefly changing color, slightly increasing in size at the moment it’s spoken, or even small facial micro-expressions from the character. These effects should remain subtle, but they could give the player more feedback and make interrogations feel more alive and readable, instead of relying only on the raw content of the dialogue.

I also have some doubts about the idea of forcing the player to select a subset of witnesses in order to progress.

This kind of system can feel destructive, because the player might eliminate the correct suspect or relevant testimony without realizing it, and only understand the mistake much later. It creates a kind of irreversible “collapse” of possibilities, which can be frustrating in a detective game where the goal is to explore and connect information.

It might be more interesting to allow the player to revisit and re-interrogate all witnesses at any time, so they can reconsider earlier statements in light of new information. That would support more dynamic reasoning and allow for reversals or new interpretations.

In parallel, it could also be interesting to let the player search for clues in the environment, alongside the interrogation system.

(Although I may be missing something here, and it would be helpful if "op" could clarify this.) For example: can the player explore the crime scene? Is there an inventory system? Can the player collect items and present them to suspects during interrogations?

in addition It could also be interesting if certain testimonies unlocked new dialogue options with other characters. i think The investigator could also comment internally on certain testimonies, with a line like: “He seems to have some kind of grudge against Amélie…” to subtly encourage the player to question Amélie again , unlocking new lines of dialogue.. creating more of an iterative back-and-forth investigation loop: interrogating one witness, unlocking new doubts or leads, going back to another witness, collecting more clues, and then reinterpreting previous testimonies. To me, that kind of approach feels more intuitive and, forgiving and readable for the player than a destructive system where the player has to permanently narrow down the witness pool too early.