Quicktime autoplay problem

I have Quicktime player 10.4 installed. Since the last update to iTunes, whenever I download a video, it automatically starts playing it. Is there a preference that is not apparent to set to not autoplay?

Can you be more specific regarding your question? (I.e., What is the video source? What do you mean by "download?" In what app are the videos being played?) There are many possible answers here depending on your specific workflow.

For instance...

1. The QT 7 Player has a preference to "Automatically play movies when opened" but the QT X player does not.
   
2. Video files can have "Presentation" settings to "Automatically play movie when opened", "Enter fullscreen mode when opened", "Automatically close after playing", and/or "Automatically quit after playing" embedded in them which the QT 7 Player will use for playback but the QT X player will ignore.
   
3. In a similar manner QTL files have "Autoplay", "Fullscreen", "Loop", "Quit", and "Kiosk" presentation options when operating under the QT 7 embed structure but won't even open in the QT X player.
   
4. On the other hand, browsers can use "FastStart" flags in the file or website programmers can code the web page with an "Autoplay" setting that automatically initiates playback of online content.

In short, there are many ways a video may begin playback automatically and only a few may allow the end user to override this action depending on how the content is created and/or distributed.

This happens when download video files through Safari. Once the download is complete, QT is launched and begins playing the file.

Again your information is non-specific. If the download is pre-programmed by the website, then download processing instructions may be telling your system to automatically open the file and begin playback. (E.g., Apple used to do this with their trailer download website option.) On the other hand, if you are manually downloading a file from its URL, the file should not automatically load in the player but will play according to the presentation instructions embedded within the file itself and as limited by the features of the player app in use. A dedicated third-party capture/download app will behave in the manner the software maker has programmed software to work which means it may or may not support user preferences regarding what to do with the file once it has been captured, downloaded, and/or converted on your system. Add-ons to Safari would likely work in a similar manner if available.

I don't know how to deal with something that the publisher has embedded (unless I can find that preference). I tried the QT7 player and set the preferences. But the newer QT player doesn't have any preference tab that I can find. Can you help with that?

No. QT X has no user preference options. (Apparently Apple considers none are needed and/or users might find a way to create encoding/playback issues if left to their own devices.) Without more specific information from you regarding what you are doing and how you are doing it, I cannot help further. For all I know you may be trying to say that your are downloading M4V files that you expect to open in iTunes but they opening in the QT X player instead. (That, at least, would be easy to fix.)