Western style arrangements typically consist of a free form assemblage of flowers in a container. In contrast, Ikebana is more of a disciplined form of art, governed by well_defined rules, principles and techniques. For example, certain precepts regulate elements like the light and the shadows. There is also what is known as the three points of balance _ shin_fuku_tai in Japanese _ which symbolize man, heaven and earth, respectively, and that have to be in harmonious equilibrium in order to give true life to an Ikebana flower arrangement. This and the fact that stems and branches are used in addition to flowers, are perhaps the most obvious differences with western_style flower arrangement.
You have all the time to rearrange or polish your flower arrangement. The flower design should look beautiful and artistic enough before you place your filler, when the filler is already placed, it would be harder to move your things around the area or place. Bear in mind that The filler are very fragile.