In a novel, you as the writer can tell us just how blue the sky is, how the heroine's sweater is only 12% real cashmere, how seeing Mr. Tall Dark-&-Handsome makes her heart race like a horse at the Kentucky Derby, and how she looks down to the last freckle.
In screenwriting, you can do just about... none of this.
For one, with a very limited page count at your disposal, space is precious. Secondly, Directors, Casting Directors, and Actors don't like to be given too much, well, direction, on what they can or can't do or be.
Different kinds of description can be found in Action Lines (what's happening), and the initial Character Description. Also very very sparingly in dialogue, which will be in a parenthetical. (But more on that later.)
CHARACTER DESCRIPTION
This generally one-to-two-lines-max opportunity to first describe a character is extremely important and is worth the effort to ensure proper execution. It's how Readers remember and distinguish this character from the rest, and how an actor decides whether or not this is a role worth taking.
You are in love with this character, and you know everyone else will be too. You know what he looks like, how he has a scar on his knee from a freak dodgeball accident, that his parents divorced when he was seven, and that he is a cat person and not a dog person (allergies, obviously...).
You've got a great handle on who this person is and a ten-page bio to prove it. Great!
Just don't include all of that in his character description.
First off, it's not necessary. If you write this character well, we'll get an understanding of who he is.
DESCRIPTION DANGER: The main reason you don't want to put too much emphasis on physical details in your character description is that, while you might see a 5'2" blonde pin up model, you may be missing out on real potential talent just because they don't "fit the bill"... and again, Casting Directors. They know what they're doing -- trust them.
The only exception to the rule is if how they look is actually important to the plot. (A tattooed heroine to fit your title, an eight foot tall man in a movie about a modern day giant, etc.) If it's not critical, cut it.
They want the CliffsNotes, not the Encyclopedia Britannica.
Character Description Example #1: Jane Doe, 27 1/2 + 3 Days -- all natural red head with extensions, super hot, determined, works at a pet rescue and adoption agency, is always the bridesmaid and never the bride, just passed up for a promotion, recently dumped by jerk face Craig Watts -- oh, and she's a vegan.
That takes up a lot of space and yet doesn't tell us much about her. Let's try again.
Character Description Example #2: Jane Doe, 27 -- although not so lucky in love or in moving up the corporate ladder, she's got the tenacity of the pit bull pups she rehabilitates at the animal rescue, and one can do attitude.
That one was better, wasn't it? And honestly we could trim this one down too, but it'll do for now.
We can run into "jerk face Craig" later (maybe at a vegan restaurant?), and show her putting away her animal rescue shirt next to a bunch of non-refundable bridesmaids dresses in her closet. All of these character bits pack more of a punch when we can actually see them in action, anyway.
SPEAKING OF ACTION
Now for the part description plays in action. While it can be tempting to give everyone a play-by-play on what's happening -- or swing-by-swing if you're describing a fight sequence -- it's best to leave at least a little to the imagination.
Action Example #1: Brett swings at Charlie, hits him right under his chin. Then Charlie punches Brett. Then Brett almost slips, but doesn't. Brett whacks Charlie with an unplugged bug zapper.
Action Example #2: Brett and Charlie grapple, on relatively even terms. They exchange blows, and Brett uses a near fall to find an unconventional weapon -- a bug zapper. SMACK!
Just remember to keep things clean and simple, and soon you'll be getting your descriptions just right. (Just avoid the bears.)