Once you've made a couple of calls, the bottom overlay will start populating with your most recent contacts. (You can't see me now, but you will in a while.) If you're a geek like me, the first thing you'll look for are the settings.
Back to the main interface, once you tap on Once the call launches, like any video calling service, your front camera view switches to a small window and the rest of your screen is overtaken by your recipient's stream.
Duo's calling interface only shows a couple of buttons overlaid in circles on top of each other on the bottom left: And there's obviously a red button to end the call in the middle bottom of the screen.
While using the app, I've found this interface intuitive and quite easy to get used to.
Duo might not do more than one-on-one video calls, but it has implemented the essentials of that pretty well with mute, multiple camera support, and even a viewfinder switch to let you gaze upon yourself for a while if you need to.
You can set the phone to vibrate or not when receiving a call, limit mobile data usage, unregister your phone number in case you're switching numbers or don't want to use Duo anymore, see and manage your blocked numbers, and activate one of Duo's unique features: Knock Knock.