Camera Backgrounds
The ability to add a video or still image as a background to a camera is a standard feature of Blender. Thanks to the controls introduced to the Camera Backgrounds shot panel it becomes possible to associate them with the shot itself.In addition to some specific operators to fine tune them, this also means that videos used as background will preserve their start time and offset relatively to the shot start time, even when the shot time range is modified.
If the media is a video, it is also possible to import its sound. It will be played all along with the shot.
Adding a video background to a shot
Select the shot and go to its Camera BG properties panel.
If this property panel is not visible, you have to turn the feature on from the Feature Toggles panel.
Then press on the “+” button and pick a media on the disk.
In the case of a video, it is possible to link its start to the start time of the shot, to make it relative. This ensures that when the shot is moved in time the video will still be played the same way. This is the default behavior. That may not be want you want though, if for example the content of the video has to match the action in the scene. In this case, just uncheck the “Link BG to Start” checkbox.