Here's an excerpt from my recent book, "The Editor Devil's Guide to DIALOGUE":
Foreshadowing is a great tool in dialogue to establish believability.
The reader sometimes needs help to swallow whatever truth is coming. By dosing information in small amounts along the way, such as through foreshadowing dialogue, the reader can “work up to” the truth.
For example: An old lady who kills her only son. Maybe’s a petty thief, but it’s still hard to swallow that she’d kill him. But easier to swallow if the reader sees the slow degradation of their relationship in dialogue.
You can move from friendly “Love you, ma” to more tense “Why are you always sweating me, ma” to threatening “Remember what happened the last time you took that tone with me, ma.” Maybe even to a conversation hinting at the abuse he experienced while she turned a blind eye.
Now we’re foreshadowing two plot elements that could come to pass: he’s going to explode, or she has to stop him before he does. That's good drama and great tension!
So think of it this way: any element of your story that would be “too large to swallow” in the opening should be foreshadowed and layered until it’s revealed. By then, the reader will expect and accept it.
But be careful: too much foreshadowing and the reader will feel they’ve already read the whole book.
The Editor Devil