I work very hierarchical, and make synopsis for the whole story, then for the 'chapters', then for the sections and so on. Somewhere down the road I shift to images, aka storyboard. 

Then I start timing (spreadsheets are handy), bottom up, then top down, and so on till it fits. Since I use existing music, that's a main timing driver too.

Poser in lowres with 1 shot per sec is a great storyboarder, and by using a slide-show at 1 fps, I get quite a good pre-viz before anything hi-quality kicks in.

15 mins is quite long though. 

Tip: 3D World recently issued a list of 100-best shorts. I've put it on my website as well, linking to the shorts themselves. Could be great examples, maybe.

Have fun.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though