Some of the worst advice that screenwriting professors give, and yes, even from those prestigious UCLA teachers who haven't had a produced credit in two decades, are the strict calls to leave only the bare minimum of what is necessary in your descriptions. If it can't be seen nor heard, they say, throw it out.
Now before I get a bunch of backfire, let me clarify. Yes, they emphasize this for a reason, since one of the most common mistakes for people entering the field are to go on overly drawn-out and unnecessary stage descriptions. (See: http://www.gointothestory.com/2010/11/q-professional-script-reader-dc-mar_28.html)
While their basic principle is understandable and for the most part true, the tenacity by which they preach the message does not reflect the growing trend in scripts over the past ten years towards a more novelistic form of description; Still succinct, not overwhelming the white space, but certainly not bare.
Rich writing can especially be seen in spec scripts, where being able to grab the "reader's eye" and bring them into the world of the story is as important as anything. Take this excerpt from "Air Force One":
"Papers containing NUCLEAR WAR STRATEGIES and MISSILE LAUNCH CODES slide into the hungry Shredding machine. Perkins manages a slight smile before he keels over dead, his duty fulfilled. The shredded remains of the nuclear football rain over his head like tickertape at a hero's parade."
He could have instead just written:
"Papers containing NUCLEAR WAR STRATEGIES and MISSILE LAUNCH CODES slide into the shredding machine. Perkins manages a slight smile before he keels over dead. The shredded remains rain over his head".
The writer, Andrew Marlowe, certainly didn't NEED to include all of the aside in his description...BUT IT'S FUCKING BEAUTIFUL. Better than most descriptions in novels. It evokes a clear and profound vision that you can instantly imagine on screen and enforces the heroic undertones of the script. And this is the type of writing that many screenwriting professors tell you to avoid! While on one hand, they are making sure you avoid excessive prose, I have NEVER EVER read a produced script that didn't have the same image-enhancing prose in their descriptions.
Reason #823 why reading produced scripts is more beneficial than relying on "teach you how to sell a million-dollar screenplay" lectures.
When to push the limits is a knowledge that becomes innate to professional writers. If you are wondering how to recognize that fine line between "unnecessary, throw this in the trash" description and "wow that was illuminating", the only way is to read scripts. Read read read.
Oh and as promised, the greatest punch-line of all time: