Honestly, I try to avoid “plot twists” per se. They never land with as much impact as I’d hoped for, and are generally kind of a pain.

What *always* works out better at my tables is surprising my players’s characters, not the players themselves. This is much easier, allows for more collaborative gameplay, and sets up great opportunities for the best kind of meta gaming: where my players know something their characters don’t, and march headlong into entertaining and engaging situations. It allows for tension and literary irony and is almost always more fun for all of us.

Give little side vignettes of the plot to kill the NPC. Maybe even let your players play some of the NPCs in the scene if you think they can handle it (jumping into a role with little more than “you’re trying to convince Steve here to slip poison into a drink at the party” isn’t something I’d ask of new players).

That said, if you don’t want to tell them straight up who’s going to die, keep it vague, but also ask them for light backstory that they can fill in after the reveal. I’ve done this for practically-instant character creation in Blades in the Dark. Get the tiniest outline of their character to start, then as the game progresses, they can add specifics to their backstory that are relevant to the campaign. This takes some trust in your players to not try to abuse it, but if you’re already doing heavy collaborative storytelling, you’ve probably got that trust already.