If you don't have pre-filmed video imported from iPhoto, your camera roll, or the Camera Connection Kit, tap the camera button to capture a scene: The app will perform a three-count, then film however many seconds of video is needed to fill the scene.
Once you've added video to your project, you can skim through the storyboard by tapping and holding until the red skimmer shows up. If you've used a longer clip in a scene and aren't happy about where you've begun and ended that scene, tap that clip to reframe.
Sadly, you can't customize the trailers beyond filling in these text and video blanks--it's more like a Mad Libs for short film, rather than any sort of fully modifiable template. That works to your advantage if you just want to cut together something short and sweet, but it's a little more annoying if you like to control your edits. I will say that despite that, I had a lot of fun putting together a Saul Bass-inspired trailer in just under half an hour. The app makes it very simple and entertaining--sometimes, it's just a little too simple.
As with the rest of iMovie's operations, I found it perfectly serviceable to cut together trailers on either the iPhone or the iPad, though the iPhone's smaller screen makes adjusting text within the storyboard a little painful. Not all devices will be able to enjoy the joy of putting together a trailer, unfortunately: The feature is only available for the iPhone 4 or later or iPad 2 or later, presumably due to graphics and hardware issues.
Extras on the set
While the trailers are fun enough to play around with, iMovie also received a few other tweaks in 1.3: a new icon; a few graphics changes, including a revision of the undo button (it's been switched an arrow to better reflect the buttons in GarageBand and iPhoto); a button to preview audio within the audio browser; exporting and playing 1080p video; and a new swipe up gesture within the timeline to create a freeze frame.
And though this isn't in iMovie proper, GarageBand now offers an "Export to iMovie" option when creating a song, allowing you to provide a custom score for a project.
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