In writing, rationalisation is everything. Having the reader accept and understand the why behind actions helps the reader suspend disbelief and stay connected to the story.
Early foreshadowing and plants of information mean that characters are able to act in a manner some might consider odd or in extreme cases too stupid to live. But because the reader understands the reasoning, they accept.
For example, in A Noble Captive, I needed a horn to be blown at the end in an attempt to summons the god Neptune. I also needed readers to be aware of its significance and what help was expected. And I needed the hero to have the knowledge and to be able to use it. So I had to weave it in through the story, but hopefully in not such a way that it was telegraphed.
One of the parts about Dan Brown's Angels and Demons that I disliked was how he telegraphed that the hero was going to need something. Little did he suspect that in a few days such information would come in handy... or words to that effect.
An example of a less telegraphed plant is in the movie Romancing the Stone, before the heroine Joan Wilder leaves to go to Columbia, her editor tells her that her books do very well down in those macho countries. Then later when Joan encounters the drug dealers, it makes sense when the man says -- the Joan Wilder. A throw away comment early on allowed the plot to move forward later.
Many times, planting ideas/abilities/beliefs in the early part of the story enables other things to happen later. These can be planted as you write the story. Or you can go back and weave it in. Sometimes when you are writing, you have to use backward logic. In other words -- ah, I need this to happen, but how can I make sure that the reader is not surprised or taken back or thinks the characters are too stupid to live (or the converse too intelligent to be believable).
Care should be taken that if you do plant something in the reader's mind that you use it in the story. The plants help define the story's world and its rules. If you make one rule early on, and then you decide to break it., you do need to give an explanation why. For example, if you have stated that your heroine never ever goes out at night without a torch in the beginning, and she does. You have to show the reaction, and the reasons why. Otherwise the reader is left scratching her head.
The key to plants is the reader remembers. Details are important. But they should be done subtley rather than hitting the reader over the head with it.
I am grateful to Swain and Creating Characters for reminding me of this fact.