In this case, when you open a video file, Quicktime will open but won't automatically start playing the file. This was an option in the old version, but now no options exist to set this.
I have found the following command to enable this feature maually:
- Open Terminal (Applications > Utilities > Terminal)
- Enter this command
defaults write com.apple.QuickTimePlayerX MGPlayMovieOnOpen 1
Your files will now start automatically.
Another option I found will researching this is you can have the file open in Full Screen if prefered also by using the follow command