Don’t delete any video clip from your camera roll until you’re certain your project is finished.
The clips you pull in from your video selection screen are NOT actually added into your iMovie project. Those clips are referenced by iMovie, and likely take up RAM, but are NOT separate copies from your camera roll.
Why does this matter?
If you delete a video clip from your camera roll, and that clip is being used in a iMovie project, that video in progress is now broken. You can double click the missing app and select another one, but you’ll also have to set up all the selections you want again. I’ve even had one project that couldn’t recover from a missing clip and I had to delete the entire project and start over. 😦 I’m a little worried that this issue was caused by deleting an EARLIER video in my Camera Roll, as I still have the video that ‘won’t play’ stored in my roll. If you’ve spent a good bit of time on a project, I’d avoid deleting any video or pics until you are finished and can successfully export the project.