Originally, L was supposed to wear comfy clothes - a turtleneck and pants, open flowy hair and a funeral hat - but it made her expression hard to read and this scene would have looked less dramatic with flowing... pants. So it came back to the dress. And Originally, this scene was much calmer. She was not as confrontational, just sitting on a mightily high horse thinking she knows everything. But with almost each line she tries to put Mukoshi down here. I use just the first Letters in the script. That the good guys are called S + M is a coincidence but they do share certain personality traits...