Wyndham asks: many scripts are full to the brim of foreshadowing - "there's a storm comin' in!" - but how much foreshadowing is too much?
Foreshadowing is often a crucial form of exposition. At its most basic, it sets-up seemingly innocuous moments or plot details that get a significant pay-off later in the story. You may want to check out Chekhov’s Gun for further clarification.
The problem with foreshadowing nowadays is that audiences have become extremely knowing and sophisticated. If you make a piece of foreshadowing fairly obvious, then the audience will be way ahead of the story in terms of how, why or when that piece of exposition is going to be paid-off.
Like all exposition, foreshadowing shouldn’t be overused or be too noticeable. There’s no real limit you can put on foreshadowing - the story should unfold as it needs to whether it’s got one or one hundred set-ups - but the key is not to feel like we’re feeling hit over the head by these flagged-up moments. It should be subtle, or clever, or funny, or dramatic, or, at the very least, put neatly into the context of a scene.
A good piece of foreshadowing occurs in Back to the Future when (** spoilers **) Marty tries to kiss his girlfriend only to get interrupted by someone trying to raise funds to fix the clocktower that got struck by lightning in 1955. It’s a good moment because Marty gets denied his kiss and the fund raiser is slightly annoying but - oh - the information about the clocktower is vital to the whole film. Better still, Marty’s girlfriend writes ‘I love you’ on the ‘save the clocktower pamphlet’ which gives Marty the all-important reminder later when he’s trying to figure out how to get back to the future (** end spoilers **). It’s all perfectly foreshadowed, and the audience recognition is strong because the set-up was clever and amusing.
Don’t be dull and flat with foreshadowing. It’s easy to give a character a line of dialogue that’s an important bit of set-up but, more often than not, it will sound too obvious or stilted. “Make your exposition ammunition” is a good way of saying dramatise what you want to get across rather than simply tell us. So, basically, if you need to set something up, do it within the context of a scene or as part of someone’s characterisation, or a clever bit of dialogue/an amusing exchange. Try to give the set-up another purpose within the scene rather than just plain exposition.