Howdy,
Check your "System Preferences" and choose Quicktime, under "Browser" there is an option to play automatically. See if you have that option selected. If you already have the movie and want to change which program opens it you can control click on the movie and a screen will show up, you can then select the "Open With" option and choose Quicktime. Hope this helps.
For others who may be interested it then gives you the option for that particular movie to always be opened in the application that you have chosen on that occasion.
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Using QuickTime as default
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