Just say "No" to portraitboxing
When recording walkaround videos, or really any videos at all, it's important to hold your phone sideways (landscape).
Otherwise, when prospects are viewing your videos, they'll see the large black bars on either side of your video (see example above). Not only that, but you're really squandering valuable real estate in the video player that could be used to show off the vehicle (or whatever the subject matter is) instead of just empty black space.
The same video, recorded horizontally instead of vertically looks a lot better, and best utilizes the space available in the player:
Make sure your orientation isn't locked
Sometimes, after recording with your phone held sideways, the video also comes through sideways, like this:
This is caused when your orientation is locked, and is specially common on iPads. So be sure to unlock that orientation before you begin shooting!