Since no third-party app lets me run my live Keynote presentation in a window the way Theater did, the next best thing is to run my presentation normally (filling the screen) and share my screen with the audience. But that blocks my view of the audience (as well as anything else on my screen). Even with two monitors, Keynote either uses the second screen for the presenter display or blacks it out entirely.
Some people work around this problem by recording a movie of a Keynote presentation and then playing it back in its own window, but that makes it difficult to time transitions properly and almost impossible to adapt the presentation on the fly — by, say, jumping to slides out of order. Saving the presentation as a PDF means losing all transitions, builds, actions, audio, and video. And although you could give someone else access to your Keynote presentation so they can run it themselves alongside a video chat, you wouldn't be able to control the timing.
Microsoft PowerPoint has an option to run a presentation in a separate window (choose Slide Show > Set Up Show, click Browsed By an Individual (Window), and click OK), which can also be used (if expanded to full screen) to fill one screen while leaving the other free for other apps. Although I prefer Keynote, PowerPoint isn't a terrible compromise for remote presentations.
Meanwhile, I want to be sure the audience sees video of me and my presentation at the same time. Lots of services display only a presentation on the audience screen, along with audio of the presenter. But for me, live video is mandatory. Some services that offer both video and screen-sharing make you choose one or the other — that is, turning on screen-sharing turns off the presenter's camera.
Options worth considering
I also found that many of the services are aimed squarely at big businesses, with prices to match. But there are numerous options that cost less than $25 per month and offer both two-way video and screen-sharing. Chris mentioned Zoom (which I also found to be a reasonable option) and the paid version of GoToMeeting (which, at $49 per month, is too pricey for me).
Of the numerous other services I've tried, here are my top contenders at the moment:
Microsoft's Skype is free for basic use, but with a Premium account, you also get screen-sharing and other features. Skype isn't quite universal, but it's pretty close — and only the presenter needs the Premium account. (A Premium account costs $10 per month, but if you plan to use it only occasionally, you can get a $5 day pass as needed; there's also a one-week free trial available.) I can show video of myself and a shared screen or window simultaneously. As long as I'm willing to use PowerPoint instead of Keynote (so I can run the presentation in a separate window, or on a separate screen), it's reasonably close in functionality to Messages Theater.
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