The script.

To begin, here is a positive review of the script by Carson Reeves at the Script Shadow blog.

He sums up by saying: "I can't stress how perfectly executed this script was. No scene was wasted. Everything was go go go. I have no doubt that the similarities to Deja Vu have thrown the chances of this thing ever getting into production into jeopardy. But let me make a plea to whoever owns this property: MAKE THIS MOVIE. Cast an up and coming actor. There are only a few locations. Very cheap to make. Then spend a ton on marketing. It will open with 10 mil but word of mouth will carry it. This can be a sci-fi classic."

Spoiler Warning: don't read the comments. However, the feedback is overwhelmingly positive.

More here.

Source Code was originally written by Ben Ripley and got a revision from Billy Ray (Flight Plan, State of Play) Script Shadow interviewed Ben Ripley on February 9th 2010.

"Why did you write Source Code?

BR: I wrote Source Code because I was discouraged with the work I was then getting. In the four years between the sale of my first spec and that of Source Code, I was mostly doing rewrites on other people’s horror scripts. I’d put a lot of effort into them, I’d get paid, and then the scripts would just sit there. I felt I had more to say creatively, and the great thing about being a writer in Hollywood, the source of our power, is the ability to generate new material..."

"Did you know Source Code was going to click with people? Were you sitting there going, “This one feels good,” as you were writing it? Or was it a total surprise?

BR: Six months before we went out with it, the Mark Gordon people knew it was going to sell. I was way too skittish to go around saying or believing that myself, but we all had a feeling the script could be something special. I should also point out that we didn’t stop with a draft that would sell. No one aspires anymore to just a development deal. We kept pushing to until I had a draft that would be made. There’s a difference, and with a spec script, you have the luxury of incubating it until it’s as strong as you think you can make it. Although I’ve written several scripts since Source Code that, to me, felt pretty strong, Source Code remains the most popular with people."

"Can you tell us what you think the key is to writing a good sci-fi script?

BR: Put character first. Don’t let the technology take over the story. Center your narrative on an emotional experience and let the science part of it be the ambience. None of the characters in your script should be aware that they’re in a science fiction film. It should be all utterly real to them."

Much more here. I do recommend reading, it's very interesting!