My first horror short story starts with a quite quiet scene, quickly escalating to the protagonist's little brother being brutally murdered, with the only thing separating a family conversation between her, her mother and stepfather, and her brother's murder, is a scream, then a quite violent scene of the primary antagonist killing the protagonist's brother. (The primary antagonist is a demon/alien that takes the form of a young child, mostly eats children but will also tolerate adults; it ate the protagonist's father in the prologue.) But this is writing, not a show. I can't simply turn on a dissonant soundtrack to amp up the suspense. I reviewed the opening chapter with a friend, and we both agreed that the scare of the killing/eating isn't enough. I need to be able to weave a suspenseful atmosphere from a mundane scene (just a family conversation) in order to create a fully terrifying start. How is this done? How is suspense created from mundane settings/positions prior to the "scare"?
Uncertainty leads to Suspense
To create suspense, the reader needs to be asking a question. They have to be guessing about ways that things could go wrong (or right!). The question could be "Is the monster going to attack the family while they eat?" or "Will the brother make enough noise to bring help?"
To generate a question like this, you need to give the reader more information than the characters. This is why people yell at the TV during horror movies - they know the "right answer" to the question, and they want the characters to pick that action.
In this case, I think you can use structure to add suspense. You can interleave sections from the monster's point of view and from the main character's point of view. Something along the lines of:
- (Monster POV) Exterior of the home. Monster must get past the dog without being attacked or generating too much barking. -- Question, will the dog save the day?
- (MC POV) Dinner Table. Quiet meal. Someone comments that the dog is quieter (or louder) than usual. -- Question, will they investigate the dog? If they do, will they find anything out of the ordinary?
- (Monster POV) Interior. Monster explores the home, looking for an ambush location or isolated victim. People get up from the table to answer the doorbell, go to the bathroom, get more food from the kitchen, etc. -- Question, will any of the isolated people be attacked? Will the monster find a hiding spot?
- (MC POV) Still at dinner. Comments about brother skipping dinner to play video games / do homework / whatever. Question, will the monster discover the isolated brother!?!
- (Monster POV) Finds, stalks, and attacks brother. Question, will the brother discover the monster in time for it to matter? Can he make enough noise to bring help?
The use of the monster POV ensures that the reader has enough information to be asking questions. The uncertainty around the answers is what generates suspense. Sometimes the characters even choose the "right" answer, and the reader gets a moment of oh, everything is going to be ok - right before they are proven wrong.
So maybe the monster knocks over something in a different room, and the family investigates. It looks like the monster is going to have its cover blown, but the characters explain the noise away and go back to the meal.
For a moment, though, the reader thought they would discover the monster and they could fight it off as a group. This little release lets you build the tension back up with more questions down the road.
To add to Jesper's answer (which is great), I'd also add that your use of language can influence the scene.
For example, eating a meal is devouring another organism, or organisms. Cutting a steak is slicing through the body of another animal, while blood and juice pour through the open gashes in the muscle. There's all sorts of metaphor work you can do here to unsettle the reader, and it seems to fit with your bad guy.
Maybe, subconsciously your characters know something bad is going to happen, there's a sense of creeping dread. It not anything you can put your finger on, but you can use your words and their thoughts to create a disturbing backdrop (just like the soundtrack in a film). Obviously how far you take this depends on your style - look at someone like HP Lovecraft for example - people can go for a nice walk in the park and somehow it's tense.
I've always thought of suspense as much like dramatic irony. The reader knows the monster creeps through the mansion, but the characters are unaware. The reader can see it as it attacks the brother, but the characters are unaware.
If you are writing from 1st person, as I often prefer, then I would add lines like, "I heard some screaming from the distance, but didn't think much of it." This shows it more indirectly.
Suspense is making the character ask questions. The first one gives the idea of 'Will they realize (in time)?" while the second one gives the question, "What's the screaming?" The brain doesn't like not knowing, and it's this that gives the suspenseful fear that people love.
Start the scene with the brother involved in the conversation and excusing himself to do something... it should be mundane like "get a book" or go to the bathroom... or... something else. Point is the reader should have an idea of how long it takes. Exit brother. The conversation continues and goes on and on for some time until someone realizes something is wrong... the brother should be back by now. It's been too long for him to take care of the thing he excused himself from doing. The protagonist leaves to fetch him, thinking he's goofing off only to find the bloody mess of the scene (Everyone knows you don't show off the monster until well into the story. No one saw "Jaws" until an hour into the film.).