I would do it in an ACTION LINE like this...(The format might be a bit whacked because of how posts work)

EXT. BEACH - DAY
It's a bright beautiful day at the beach. Bob, Frank, and Burt are the
only ones at the beach.

BOB
Blah, blah fankin banking!

FRANK
Shilo, What for.

BURT
Freakin deaking...

The three grow silent. They look away from each other not knowing what to say.

Myfilm"Shadow of Crime"
Trailer:
http://www.vimeo.com/4103913
Where to buy:Shadow of crime
https://www.createspace.com/288191

Thanks Rjames, I appreciated that!

Also when you introduce main characters and supporting...do you have say what they are like i.e.

Dan enters the room (a 28 year old steal, worker, white)

I mean do you have to go in so much detail? I mean do you have to state the ethnicity as well? Thats my main question

Pretty much the same. Your characters will always either enter the screen from some direction, or a new scene will begin with characters in them. Either way, this will be described in an 'action' line:

?code?
INT. VIDEO WORK AREA LABORATORY DAY

The video pirate operating this equipment is MARTY McFLY, 17, a good looking kid who has
an air of confidence just shy of cockiness. Hes wearing a silver Porsche jacket, and
like most typical modern day kids, not a stitch of his clothing is without some brand
name or form of advertising. Hes looking at an ad for a guitar amp in ROLLING STONE.
?/code?
or
?code?
PERU
1936
A narrow trail across the green face of the canyon. A group of men make their way along
it. At the head of the party is an American, INDIANA JONES. He wears a short leather
jacket, a flapped holster, and a brimmed felt hat with a weird feather stuck in the band.
Behind him come two Spanish Peruvians, SATIPO and BARRANCA. Bringing up the rear are five
Yagua INDIANS. They act as porters and are wrangling the two heavily-packed llamas. The
Indians become increasingly nervous. They speak to each other in bursts of Quechua. The
American, who is known to his friends as Indy, glances back at them.
?/code?

The amount of detail will depend on how important it is to the character. Most screenwriting courses (and books) recommend keeping detail to the minimal, most essential elements that are really necessary for the character, which then leaves your director (and casting director) much more room to successfully cast it. If you define your character so precisely (180cm tall, fit, about 80kg, light blond, handsome but rugged, etc), the (casting) director may get stuck with that image in his mind and might pass on a very gifted actor with a bit darker hair who would otherwise fit just perfectly. Also, when trying to describe the character, you want to provide details that will describe the personality (in first example from Zemeckis, it is a confident kid who wears hip, modern clothing; in the second one from Kasdan, all we know is that the guy is wearing a leather jacket, a holster and a hat with a funny feather). Avoid actually describing the physique, unless it is THE key quality (a drop-dead gorgeous knock-out, for example):
?code?
All three are still free-falling.

James Earl Jones nods at the Skydiver who, even under the
helmet and goggles, is clearly a beautiful woman.

Meet ALEXANDRA "ALEX" MUNDAY, one of Charlie's Angels.
She's a sultry bombshell -- a classic femme fatale --
only she's playing for the good guys.

She gives James Earl Jones a wink, then jerks Pasqual's
rip cord and -- WHOOMPH. Pasqual's chute deploys. He
flies upward, leaving --

Alex and James Earl Jones, both still falling. Below the
ocean screams up at them. Not much time left.
?/code?

If those things are essential to the story then you should include them.

For example, if Dan could not be a black man because of the story then
you could mention his race. But only if their might be some confusion
while reading. If Dan could not be 24 or 34 due to the story then you
should mention his age. Or if it's not clear by reading the script. I
wouldn't put the characters occupation in the description. That's
something that should come out in your script.

I suggest not putting anything in parentheticals about the character
unless it's; 1 - essential to the story and 2 - not clear while reading
the script.

=============================================
The aim of an argument or discussion should not be victory, but progress.
Joseph Joubert, essayist (1754-1824)

I do it like this:

INT. BINGO HALL -- NIGHT

BETTY(50's)she's wearing cheap off the hook clothes with perminent sweat rings around her armpits. She wipes sweat away from her forehead, and concentrates on the five Bingo Cards in front of her.

Myfilm"Shadow of Crime"
Trailer:
http://www.vimeo.com/4103913
Where to buy:Shadow of crime
https://www.createspace.com/288191

I've heard actors want a bit of personality in there, something for them to get their brain around. That could be almost anything. It really isn't the age or ethnicity. On the other hand the age and ethnicity gives a reader a visual to grab onto. That could mean the difference between a go picture and a screenplay being tossed aside.

RJSchwarz

And if the age or ethnicity isn't essential to the story it could
mean the difference between a go picture and it being tossed
aside.

Many times a script will be sent to actors of different ages and
races. If you write a protagonist as a 30's white man, but the
story doesn't really NEED to have a 30's white man, the producer
might not think of Denzel Washington so won't send it to him. If
the producer does sent it then Washington might not even read it.
Of course if the story is "American History X" Washington won't be
playing Derek Vinyard but Eli in "The Book of Eli" could have been
played by a 30 year old with man or a late 40's black man. The age
or ethnicity really isn't important to the story. I know, I know;
Eli couldn't be a 15 year old but he could have been 25 to 65 with
just a line change or two.

=============================================
The aim of an argument or discussion should not be victory, but progress.
Joseph Joubert, essayist (1754-1824)

For a pause, I often use BEAT. You can use it in the dialouge, if a person pauses before finishing his or her sentence.

For introducing a character, always cap the characters name, and describe him or her how ever you would to someone else. Focus on the important things, that will be relevent for the rest of the story.

" see things through my eyes "

I think age is very important. It's your story though, tell it the way you want, let the producers worry about giving a SP to an actor after the SP has been Greenlight. Also a brief description of quirks and how a character appeaers is very important, so a reader can visualize the story. I let the story bring out the character. Good actors will get the character down by learning personality as the script developes -- they won't gain much from a detailed description in a action slug line.

Also a good tool to use when writing a script. Think of each character in the SP, and come up with a brief history, and personality for that character, and stick to it through the whole SP. MOVIES ARE GOOD CHARACTERS, IN GREAT SCENES, THAT MAKE EXCELLENT MOVIES.

Myfilm"Shadow of Crime"
Trailer:
http://www.vimeo.com/4103913
Where to buy:Shadow of crime
https://www.createspace.com/288191

In addition to all the advice already given, I highly recommend the following two resources to any SERIOUS aspiring Screenwriters:

http://www.amazon.com/Film-Scriptwriting-Practical-Manual-Second/dp/0240511905/ref=cm_syf_dtl_top_3

http://www.wordplayer.com/

Brian Dzyak
Cameraman/Author
IATSE Local 600, SOC
http://www.whatireallywanttodo.com
http://www.realfilmcareer.com

Much better post for script writing tips thanks! For sharing your experience

The wordplayer.com forum has massive amount of incredibly useful advice on everything related to writing good script, including avoiding scriptwriting mistakes, clichés and bad dialogue, etc. It is free, so all you need to invest is a lot of time to read it.

There are two things to think about when formatting - the message you're trying to convey and spec guidelines.

If you're writing on spec I suggest checking out "The Scec Scritp Format Guide," or Dave Trottier and Chris Rily both have some good books on formatting.

As far as what detail to include, that really depends on how essential the information is. Is it important to take note of what the character is wearing or what race they are? If yes, then it must be included.

Jamie Paszko
Publisher/Editor
www.FilmSlateMagazine.com