Getting Started

Installation

Download the .dmg file of the latest release.
Open it.
Copy the QuickLook Video app to your Applications folder (or Applications → Utilities if you prefer).
Run the app from where you copied it.

Alternatively, if you have Homebrew installed:

You don't need to keep the app open - you just need to run it once to make macOS notice the app's QuickLook and Spotlight plugins.

To see thumbnails of video files you will likely need to press the Refresh button. This tells QuickLook to clear its thumbnail cache and then restarts Finder to make it refresh its thumbnail cache. Alternatively you can relaunch Finder (hold ⌥/option, right-click on the Finder icon in the Dock and choose Relaunch) or log out and back in again.
You can adjust the Snapshot setting to change the approximate moment in the video at which a snapshot is taken for the thumbnail image. Irrespective of this setting, the snapshot will never be taken more than half way through the video. Press the Refresh button to generate thumbnails with the new setting.
You may experience high CPU and disk usage for a few minutes after installation while Spotlight re-indexes all of your "non-native" audio and video files.

The QuickLook "Preview" function displays one or more static snapshots of "non-native" video files. You'll need a media player app (e.g. VLC or MPlayerX) to play these files.
Interlaced content is sometimes not de-interlaced in QuickLook thumbnails and previews.
Requires macOS 10.13 "High Sierra" or later on a late-2013 or newer Mac. The last version to support hardware older than this is Release 1.99.