Getting Started

Installation

Download the latest release.
Double-click on it.
The Installer app will walk you through the installation process.
To see thumbnails of video files you may need to relaunch Finder (ctrl-⌥-click on the Finder icon in the Dock and choose Relaunch) or log out and back in again.
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.

Alternatively, if you have Homebrew installed, you can install using Homebrew Cask:

brew cask install qlvideo

QuickLook struggles with .mp4 files. In particular, QuickLook on recent versions of OSX cannot preview a .mp4 file containing H.265-encoded video. Rename your .mp4 files as .m4v, or use another container for H.265 content - e.g. Matroska.
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 OSX 10.9 or later. Use Perian for equivalent functionality under 10.8 and earlier.

You can customise this plugin's behaviour by entering the following in the Terminal app:

defaults write uk.org.marginal.qlvideo SnapshotAlways -bool XX : Controls whether QuickLook "Preview" shows static snapshot(s) even if a playable preview is available. Default is NO.
defaults write uk.org.marginal.qlvideo SnapshotCount -int XX : Maximum number of snapshots to show in a QuickLook "Preview". Default is 10.
defaults write uk.org.marginal.qlvideo SnapshotTime -int XX : Time offset in seconds for thumbnails and for single snapshots. Unless the video clip is shorter than twice this value, in which case the snapshot is taken at the mid-point. Default is 60. This setting doesn't affect thumbnails that have already been generated - use qlmanage -r cache and re-start Finder to force regeneration of existing thumbnails.