|Optical illusion pic obtained here|
The story I'm working on now is about a haunted house. Not that anyone calls it that. It's creepy, it's scary, locals have stories about daring each other to go near it, but it's never "the haunted house." My main character, who witnesses a number of impossible events, specifically doesn't tell her housemate, out of fear she'll scare her away and then have to be in that house all alone.
In dialogue, a character's refusal to address something, or a refusal to say something aloud, can create tension. People rarely spout their life stories and all their secrets at first meeting. There are things all of us avoid talking about, sometimes because it's too personal, sometimes because we don't know enough about it, and sometimes because it involves a secret. Get two characters talking, one of whom is keeping something from the other, and the conversation becomes a dance, as one pushes conversational boundaries and either confronts the one avoiding the subject, or accepts that the other person doesn't want to talk about it.
In my own dialogue, nothing makes me prouder than to have a character give an answer that's factually correct, but that doesn't answer the intent of the question. My dishonest characters are masters of lies by omission, rather than outright and blatant untruths. It amuses me to have my protagonists teased with something so near the truth, yet so widely misleading.
Bear in mind, this is a tricky skill to develop. You can't assume that your readers all have the same cultural context as you, and therefore that they'll fill in your gaps with the same things. Nor can you assume they'll have the reading comprehension to understand you've deliberately left something out. You'll want to call attention to it, either through repetition, by subtle cues that something's missing, or by having a character make note of the omission.
Remember, you're writing in your own world, with its own rules. If you leave something out, a reader doesn't know if you've left it out because it doesn't exist, or if it's on purpose to highlight a theme.
But, if you know how to use this skill, it's an excellent way to avoid bashing your reader over the head with telling. It's going to take some practice, but it makes for much more elegant writing, in the long run.