I wonder if the player actually needs to click the NPC if this is the only option he has anyway in the store. You might play a video as soon as the player enters the store (or display a text, if the video is too much effort. In my estimate 1 min video = 1 day of work, of course depending on the amount of new assets and animations you need). You can make videos as well as play them with the Unity engine.
After video is over the player stand inside the 3D store and needs decide where to go next. You might want to provide the player with some guidance where to go next. I have done this in my detective game by footsteps on the ground using decals in Unity.
BTW if you'd like to provide a murder case as a story I'd offer to build it into my detective game. After the first section, which presents the intro for the investigation, the other sections appear in random order. Mixed in are sections to confuse the player. This might be irrelevant facts or a person lying. Finally, after the player has filed his investigation report, a summary of the murder case is presented, so that the player can see how right or wrong his investigation was.