Media Player assets by default are set to Play on Open which is why the video automatically starts playing when calling the Open Source function.
There are additional commands you can issue to the Media Player asset such as pausing, rewinding or stopping the video once it has begun playing which can be found under the Media Player section when dragging off the Media Player Reference.
In this example we elected to put our media content inside our project's Content/Movies folder. While this is not a requirement it is recommended in the event that you want to package your project, as this is the only folder that will automatically include the content as part of the packaging/deployment process.
You could point your File Media Source asset to folders outside of this location and the media will play, however if you were to deploy this to a mobile device for example, the content would not be included as part of deployment.