Play or skim video in iMovie on Mac
When you play a clip in the browser or a movie in the timeline, your video plays in the viewer.
In addition to playing video, you can skim video by moving the pointer back and forth over clips. Skimming is useful for quickly previewing video or finding a particular moment.
Play video
In the iMovie app 
on your Mac, do one of the following:
Position the pointer over a clip in the browser or timeline where you want to begin playing, and press the Space bar.
Click a clip in the browser or timeline where you want to begin playing, and then click the Play button
below the viewer.To stop playback, press the Space bar, or click the Pause button
below the viewer.
Play clips and movies in full-screen view
In the iMovie app
on your Mac, click in a movie in the timeline or a clip in the browser to position the playhead where you want to begin playback.Click the Play Full Screen button
below the viewer.
To leave full-screen view, press the Esc (Escape) key, or click the Exit Full Screen button 
below the viewer.
Play a clip or range
In the iMovie app
on your Mac, select a clip or part of a clip in the browser or timeline, and choose View > Play Selection.
Play video in a continuous loop
In the iMovie app
on your Mac, choose View > Loop Playback, so that there’s a checkmark next to the menu item.Position the pointer where you want to begin playback, and press the Space bar.
If the pointer is over a clip in the browser, the clip plays in a continuous loop. If the pointer is over a movie in the timeline, the entire movie plays in a continuous loop.
Tip: To loop a portion of a clip or movie, select part of a clip in the browser or the timeline and choose View > Play Selection.
Turn audio on or off while skimming
In the iMovie app
on your Mac, choose View > Audio Skimming.