The way i write every script i have ever done is i, first come up with the VERY basic story line which basically i have in mind the basis for the story and a few key scenes.

Then i create each character that is pivotal to the story and i completely flesh out their personality and back story, and then basically what i do is i put these characters into my very basic story and i let the characters create the story, I think this works to create a very realistic story because if your basing everything that happens in the story off of what that character would actually do then obviously your script has a level of realism.

One thing i have a problem with at times is the issue of your main characters being alike, as was mentioned earlier in the thread most of the time people who are similar in personality tend to be friends and when im writing my scripts i always want realism so i find myself writing very similar characters as friends and i find at times this can be troublesome because they all act similar but then when i diversify the group and mix different personality types as friends i think it loses that bit of realism.

So its a balance you have to hit, where the characters are similar enough to each other to be believable yet distinct enough to make the script interesting.