A quick tip if you are a Safari user, on High Sierra, but your daily life requires you to use Google Hangouts with some frequency. If you fall into all three of those camps, you’ve faced this frustration already: the hangout starts, but although the camera light comes on, your end of the video never starts.
As someone who would rather use just one browser, and (don’t tell my coworkers) finds Safari a lighter browser that gets the job done just as well, here’s a work around until the Google plugin authors get around to acknowledging and/or fixing this bug:
If you launch hangouts in Safari, then start up PhotoBooth, the process of turning the camera on by PhotoBooth will activate the camera in Safari. It’s a crappy work around, but I got it (and tested it as true) from Apple directly.
Hope this helps someone else out there, because I spent a month or more of frustration and browser swapping before finally asking. FWIW, the problem appears to be in the plugin from Google, not Safari itself (which is where I started my investigation).