Well, the weight is supported by only one foot at a time, so once you've placed the moving foot the active role of the standing foot is finished. What happens next is follow through, not propulsion.

To me, the prep step is the initial side-step taken prior to the first forward step by the man in the two waltzes danced in the movie "Shall We Dance". The first waltz is danced by J-Lo and her fellow instructor in their demo to Gere and the other new students. The second is Richard Gere's waltz in the Taittinger competition. My in-depth anaysis of the step...its cool.

IndySpinner. As you have pointed out the preperation step can be to the side if you wish. Some stand on the left foot with the right slightly behind, get a good compresion on the standing foot, and push off. With the left side being left behind it is possible to get a very intesting swing through one onto two if this is a Waltz , or onto the slow and through to the two quicks if it is Foxtrot.
Reading at the very top the question was, is the first step the most important. Or is it the second or maybe the third which is the most important.

"IndySpinner. As you have pointed out the preperation step can be to the side if you wish."

Sorry, no.

The preparation step must generally move in the same direction as the step it is preparing you for. The exception is that instead of a preparation step you could take a prepeartion rise, usually in the opposite direction, and then start the actual dance on a downswing rather than an upswing, but this is rare - generally it applies to viennese waltz and to tango where the swing is almost flat and non-existent, respectively.