I have a "how to write a script from a single idea in your head" posted on my website (
http://www.yafiunderground.com/AJ/howto.html ) which basically says:
write down the idea.
make 3 outline pieces (on paper or in a word processor):
..I. Act 1, introduce the people/places/relationships
.II. Act 2, Introduce the problem/struggle
III. Act 3, Resolve the problem introduced in Act 2
This is the standard greek story structure and has worked for thousands of years.
From there, fill in the space between each. If you write a piece in act 2 that needs another character, figure out if you need to introduce them in act one. It's a very random writing style, but as you flesh the story out, you have a complete work witout holes by the time you're done as you've filled all the holes as you were adding elements.
To make a longer movie, you make the same 3 act structure under each of the headings, and 3 act structures under each of those. As long as each of the smaller pieces is wrapped up and leads naturally into the next larger section, the movie should flow and feel right to an audience story wise.
..I. Intro
.....1. Intro
.....2. Conflict
.....3. Resolution
.II. Conflict
.....1. Intro
.....2. Conflict
.....3. resolution
III. Resolution
.....1. Intro
.....2. Conflict
.....3. resolution
Once you've got all the stuff on paper/computer, then worry about who says what. At this point it's easier to determine what the characters will say as you know what each of the characters is about and their place in the story world you are writing. Dialog comes last in this approach. With this approach, you can get to a filmable script in short order. It's mostly just walking through the logical story events one at a time.
You will want to consider that most characters need to show growth of some sort from beginning to end.
Check out
http://www.sonnyboo.com and see his "one shots" one take filming excercises. 1 minute long pieces that exemplify the versatility of the 3 act structure.
There are many other ways to write a script, but I think this way is most approachable for non-writers, and a great excercise for writers. Other alternatives will follow this post by lots of writers...they will all be very valid as there isn't one universal way to do what you're asking.