Flash ExternalInterface Does Nothing

I recently helped another developer who was trying to integrate JavaScript with Flash. I recommended using ExternalInterface and pointed him to a couple of links. He emailed me a few hours later saying he had everything linked in, but couldn’t get the example to work correctly. After troubleshooting the problem I realized the allowScriptAccess parameter was set to sameDomain. This won’t work when accessing the page from the local file system, but only when accessed from a webserver (ie http://localhost/external.htm). To get this working the allowScriptAccess parameter needs to be changed to always. This seems trivial, but after an hour’s worth of investigation I stumbled across the answer.