If the clip you want to use for an effect has transitions attached to it, applying the effect may remove the transitions (iMovie warns you if this is the case). After adding the effect, simply create the transitions again.
To change the speed of a clip, select it and choose the Fast/Slow/Reverse effect.
Some effects, such as Fairy Dust, feature controls in the small preview window. Click within the preview area to determine the angle of the light burst, as in this example (ouch).
Apply the Effect p. 61
You can't change an effect's settings once the effect has been applied like you can with titles and transitions. Instead, you must delete the effect, make your changes, and then re-apply the effect.
To stop the effect from rendering, press (period), or choose Undo from the Edit menu.
When you delete an effect from a clip that contains multiple effects, you delete only the most recently applied effect, not all effects on that clip.
If you apply some effect types to a still photo, the image needs to be converted to a regular video clip. iMovie warns you if this is about to happen, and gives you the option to cancel.
Effects cannot be applied to clips on the Shelf. However, you can add an effect to a clip in the Time-line and then move it to the Shelf for later use.
You can apply transitions and titles to clips that incorporate effects.
(period), or choose Undo from the Edit menu.