It is certainly possible to build an atmosphere of suspense and uncertainty about the plot and events in the game by describing unsettling scenes or unexpected events, using a manner and tone of voice that suggests seriousness, dimming the lighting, et cetera. A gamemaster should not, however, aspire to actually frighten, repulse, or otherwise seriously distress the players, and in a horror game or one that presents horrific or gruesome events should be aware of any issues the players may have (e.g. past trauma, personal dislikes, et cetera) and make sure the players are okay with the tone and material that will be presented.
One thing to consider is that while a lot of gamemasters seem to think that they are ‘telling’ the story that the players are in (and some even compete with the players), tabletop roleplaying should really be a collaborative effort of collective storytelling in which each of the characters is the protagonist of their personal story. Trying to trick or fool the players by, say, unexpectedly changing the genre of the game from adventure to existential horror or ‘killing’ a player only to make it all a fever dream, or similar narrative tricks that are often used in novels where the reader is a passive observer do not generally work well in TTRPGs because it essentially removes the agency of the players in controlling their characters and actively contributing to the story. It’s okay to do squirrelly things like that in a game like Paranoia because the focus of the game is specifically about subverting the tropes of standard roleplaying and literally pitting the players against one another, but in general your players should be aware of the tone and general content of adventure and agreeable in playing into it, not just puppets for the gamemaster to manipulate.
Some of the best games I’ve played have been sandbox scenarios with minimal directed plotting, where the characters are actually creating plot developments through their actions and the gamemaster has to extemporize around their choices with the elements and locations in the sandbox. In such cases, it actually helps to have the players narrate scenes or describe locations given some basic information (e.g. “You walk into a nearly empty diner after midnight; who is there and what are they doing?”) which makes them very invested in the game versus just railroading through a structured plot with an essentially pre-determined ending. It’s more fun for the players and frankly more fun for the gamemaster (provided they are prepared to work off the cuff and have done some minimal preparation to accommodate the players like having a folio of generic NPCs to draw from and adapt).
Stranger