First of, using soft shadows when making a short film might not be such a good idea. Soft shadows, especially in anim8or take along time to render, sometimes even hours every frame. Considering you will need probably hundreds of frames, soft shadows are a bit out of the option when making films.
Therefor I would stick to hard shadows, which wont render that long.
Second option could be to render a normal pass with no shadows and an ambient occlusion pass with low AA setting. Then blend these two and you will have really subtle soft shadows.
Now how to create shadows.
At first you need a light. There are 3 kinds: spotlights (like a spot), local (like a normal lamp)and a infinite (like the sun).
Since you want to make the sun, use a infinite light or maybe a local (depends on the scene a bit).
This light will have to cast shadows: go to the fixed properties tab and click advanced. Under shadows; set Casts shadows on and make them raytraced (volume shadows don't give a good effect most of the time). Adjusting the size will make the shadows softer: 0 hard shadows, 100 is very soft.
When working with the ART raytracer you don't have to say which objects cast shadows: all the objects in the scene will cast and receive shadows.
With the scanline render you will have to say this for every object in the scene if they cast or/and receive shadows.
Because of this, using ART, sometimes you scene will get black. I might have happened that sometime is blocking the lightsource (ie a window, if it's a indoorscene).
i hope this helps
