Sushi is very simple to work with. All you have to do to preview a file is to select it and then press spacebar button and a minimalistic window with the preview appears.
You can preview audio files, video files, image files and pdf and txt documents. The video and image files can also go fullscreen, while the audio and video files are offered a progress bar to allow the forward/backward function for better preview. This way you can basically even watch a whole movie without using a video playback application.
To install sushi you should search in your distribution’s package manager for the gnome-sushi package on Debian/Ubuntu or just sushi on the rest. You will have to restart Nautilus for it to work properly. Note that if you’re not using Nautilus, sushi will probably work on you other file manager too so don’t hesitate to try it out.