15 seconds is half of 30 seconds.
30 seconds isn't very long to begin with, but it seems like an eternity after you've tried cramming an idea into 15 seconds.
Clients love 15-second spots because they can buy a lot more of them for the money. In media impressions, they're not that different.
But in creative terms, they are very different. In a :30, you can set a scene. You can have some dialogue. Tell a little story.
A :15 is the equivalent of a billboard. You get one idea. A simple one.
DO NOT try to cram a :30 idea into :15. The results will not be good. I say this, and yet I've tried it many many times. I've had ideas that I just knew were so great that I wouldn't let them go, even though they were too complicated for 15 seconds. I've had :30 scripts that a creative director promised to a client could become :15s. Most often, I've had :30s that had to also be cut down to :15s because they had the in the plan. Whatever the reason, the results have not been good.
Know going in if you have to do a :15. Then concept for a :15. KEEP IT SIMPLE.
One of my favorite :15 campaigns of all time had a very straightforward, single-minded idea.