After you've tried effects by simply applying them to successive clips, you'll probably discover that you want to make things more interesting or customized.
Because you've already applied an effect to a clip, you will need to reapply the changed effect. To do this, select the clip you want to change the effect for, choose the effect you wish to change, make the changes, and then click Apply. A dialog message sheet window will appear in some cases to let you know that the new effect invalidates the previous one. You must choose OK for your new effect to be processed .
Task: Changing and Updating an Effect
This example picks up where we left off with the last task. We've decided that we want to try the mysterious Effect In and Effect Out features in the Effects palette. We want to slowly increase the impression that the effect has on our clip over the space of a few seconds by bringing in the effect to give the clip a unique feel and then fading out the effect.
When you click the Apply button, iMovie starts to process the video. In a short while, you can preview it to see the final version of the video. Of course, if the effect doesn't measure up to your expectations, you can continue steps 14, trying out different adjustments until you're happy with the effect.