I've heard that sometimes we have to include information in the screenplay that won't show up on screen for the purpose of keeping the momentum of the story.
How far can we take that? For example, if we describe a character as "clever and shrewd -- the type of guy who would get straight As and brag about how they never studied before the test", even though we never see him doing that -- would that be an acceptable character description, or would we be overstepping the bounds?
The point is this -- when a character appears on screen, he appears in a certain way, presents a certain attitude, deals with situations in certain ways as a matter of form.
Now, you can always simply grab from a list of adjectives, but sometimes adjectives don't quite give the sense of someone's personality -- and by that, I don't mean the sense of someone's personality after you've gotten to know them for a long time -- I mean the sense of someone's personality as an audience would experience it when they are first introduced to that character.
So in that situation, I don't think that there's any problem with using the tools that prose gives us to convey what an audience watching the movie experiences simply by seeing a character, which is something that naked description alone is going to have a difficult time mastering.
I don't think there's anything wrong with describing a character as, "slick as an eel" -- because in a very few words it gives a clear sense not of something that we *can't* see but rather of something that the audience is clearly expected to see as soon as they set eyes on the character, only that particular quality may be distributed in countless different ways, through dress, through performance, through the way in which he delivers his lines -- but you don't want the reader to have to wait for that accumulation of details (many of which may be more information that you'd want to put into a screenplay) to convey what an audience understands immediately when a character appears on screen and starts talking.
For instance, in a screenplay of mine I described a character as "A man in his forties, sporting a pony tail, which, like a mustache on a 16-year old, only serves to accentuate his true age."
I could simply say that he's a man in his forties with a pony tail, but the additional detail gives the reader a much clearer sense of what his character is -- a man getting older and struggling to stay young by holding onto an inappropriate look.
And in those limited circumstances, I think that it can work for you.
That is, you should always be aiming to tell us what we're seeing on screen, but you need not always be conveying that information in a literally descriptive way.
NMS