So... here's one I prepared earlier!
Many spec writers want to introduce symbolism in their scripts. And why not? Sometimes the best writing we see "hints" at other things; there are multiple ways of "reading" it -- and us screenwriterly types can pat ourselves on the back for "seeing" it.
When it comes to symbolism of any kind, think LAYERS, like an onion. You can do this any way you want: visual metaphors, allusions, motifs, character traits. There are no rules, remember. However, the biggest issue I see when it comes to symbolism:
It's not clear what the screenwriter is actually doing.
Let's go back to that notion of an onion. It's round, right? It looks like an onion. Well "durr" you say... And this is how your script should be: it's a script, it's got a story, we should know what the story IS - however you structure it, linear or non-linear. So if you have a story about a big huge alien eating everyone's ass, that's what your script IS: its top layer, if you will.
However, too many specs are so into the symbolism, they forget about keeping the plot simple. Taking into account a produced film then, ALIEN is as I've just described. A monster gets on board a ship and starts picking people off, one by one. Easy to comprehend, just like it's easy to look at that onion and say what it is, regardless of what language we're speaking. Basically:
EVERYBODY SHOULD BE ABLE TO GET YOUR STORY AT FACE VALUE: that top layer is plot, pure and simple: "in this story [this event or thing happens]".
So, you have the "top layer" ANYONE can get... so now let's look at the NEXT layer, those all-important characters. Very often scribes want to have SYMBOLIC CHARACTERS - representations of good and evil; God and the Devil; Adam and Eve; happiness and sadness; commerce and art; communism and capitalism and so on (and yes, I've seen all of these, many times). And why not? But too frequently they're so in love with these representations and what they mean in the grand theme of their work, they forget to essentially write the characters as actual people and audience can relate to.
So let's look at ALIEN again. But let's step aside from Ripley; she is NOT the catalyst for the events that happen in that first movie, it's not actually her actions that kick everything off -- instead, it's John Hurt's character, Kane. An English guy, very curious, loves himself a bit, thinks he's better than the rest of his crewmates, so the same rules don't apply to him. We even see him wake up first from hypersleep, even though he's not the protagonist.
That's Kane on the surface... but Kane underneath? Check it out:
BECAUSE he thinks he's better than the rest, because he thinks the same rules don't apply to him, he disobeys orders - AND TAKES EVERYONE INTO THE PATH OF THE CREATURE... but not only that: look at his name!! Kane = Cain. What does Cain stand for from the bible? DEATH - he is a murderer. Whilst Kane is not a murderer in ALIEN, he DOES bring death into the ship. Also, In the dictionary, to "raise Cain" is to "cause a commotion". Without Kane, there would be no movie cos they'd have never come into contact with the facehugger.
But guess what? You don't need to get Kane's name is like "Cain" or anything else to appreciate what Kane *is* in this movie - which is the doom of everyone else. That second layer is just cool and good writing.
So in other words: don't be too clever when it comes to symbolism. You need the "obvious" stuff first ANYONE can get... Add the fancy stuff in that second layer for others to get... like film students and screenwriters.