The importance of your characters ranges from your hero right down to soldier #11.
But regardless of the importance of your characters, when you introduce them you need to write their name in CAPS.
When a reader reads your screenplay, at best they read with a 70% focus. The majority of readers have a hell of a lot of screenplays to get through, they don't have time to hang on your every word. They speed-read and get a feel for if your story was clear, the characters popped, the concept is sellable etc...
This is why we put character names in CAPS the first time we introduce them. This signals to the reader that this is a NEW character.
Let's start with how to introduce your important characters.
When a character is a main player, it's better to give the reader more information about them.
When you introduce a character in detail but then that character only has one line and they disappear from the rest of the script it can be confusing for the reader.
Don't waste time introducing un-important characters in detail.
When introducing your hero, you need to cover three aspects -
Physiology - what they look like. Are they strong, are they weak, are they tall, hunched? Give us a visual picture of them. But don't just list their physical traits. Try to be inventive and creative with the way you describe them. Rather than saying John is fat, you could write John substitutes exercise for eating and his waist-line shows it.
Remember, your job is to entertain the reader as well. When you write dull, dry sentences the reader will start to check out. The more interesting you can write a sentence, the more engaged your reader will be.
The next is SOCIOLOGY - where your character fits into society. Are they a queen, a beggar, a royal Knight, teacher etc...
The next is PSYCHOLOGY - their state of mind. Are they bi-polar, are they Zen and calm, are they quick to temper, etc...
You want to get these three aspects of your character across in one sentence.
After you write your character's name in CAPS, you then put their rough age in brackets - i.e... JOHN WILLIAMS (40s) - or MICHELLE JONES (20s).
You can put a specific age if you want - such as JOHN WILLIAMS (43) - but most actors play an age range that is roughly 2 decades.
Only write a specific age is it's relevant to the plot.
The other exception is when writing the age for under 20.
There's a big difference between an 11-year-old and a 16-year-old.
Likewise, a 3-year-old and a 9-year-old.
When you introduce characters that are less important it's best to keep their introductions minimal - focus on their function, as that's the most important aspect of their character introduction.
LUCY RHODES (30s), a paramedic, is the first to the scene, she pulls the glass away and...
If there is something important for us to know about Lucy's physical trait, then you could describe that also...
Perhaps Lucy only has one arm. We need to know that so tell us.
When introducing less important characters only tell us what we need to know for the story to make sense.
SMALLER CHARACTER NOTE...
While it is inevitable that you will have smaller characters in your screenplay, just because they only have one or two lines doesn't mean they should be any less developed.
One way readers separate the pros from the rest is how developed smaller characters are.
If you can get a hold of the screenplay - The Disciple Program - by Tyler Marceca - this script is an example of incredibly well developed minor characters.
A lot of amateur screenplays have too many minor characters.
I suggest you go to the screenplay you're working on at the moment and do a pass that just focuses on the smaller characters.
Your first step is looking to MERGE minor characters.
Often there are three small characters all serving the same function.
Why not merge them into one character and develop that character a little more.
If you have one minor character that has six lines, rather than three small characters with two lines each, that merged character with the six lines will feel more real and more developed.
Once you have gone through and merged as many of your minor characters, then spend a day going through and developing each of your minor characters.
Try to give each their own special attribute, something that makes them feel like they are a real character, not just something thrown in for the purpose of serving the plot.
Imagine that character's OWN STORY.
You don't need to go into too much detail, but give thought to who they are and what makes them tick.