A live demonstration slide needs more than a working camera somewhere on the Mac. The object on that slide must use the correct source, show the intended content and appear when the demonstration reaches it. Establish those facts with harmless material before the session.

List the expected source for each demonstration: the Mac camera, an external camera, or a connected device’s screen. Name the source according to its role so a later presenter can distinguish it from another connected device with a similar model name.

In Keynote on Mac, Insert Media, Add Live Video initially adds the Mac camera. To configure another source, select the object, open Format, Live Video, Source and the add button, then choose and name the source. Apple requires external cameras or devices to connect to the Mac by cable. In a multipresenter slideshow, available live-video sources belong to the host computer.

A connected iPhone or iPad can supply its screen. Inspect the actual source preview rather than assuming the device name means its camera view. The Source menu also lets an existing live-video object switch among sources already configured for the presentation.

Rehearse the exact demonstration view

Use an unmistakable test subject or a harmless screen containing a large test label. Start playback on the slide before the live demonstration, then advance to it. Confirm which source appears and whether the important area fits inside the object’s visible frame.

Check that the source content is large enough for the audience to read. A phone screen that looks clear while editing may be too small after sharing the presentation window. Ask the receiving viewer to read one representative label without coaching.

Outside playback, View, Show Live Video Preview provides controls to turn the source on or off. Rehearse that control with the test content, then return the source to the state required for the beginning of the presentation.

Plan the host and fallback together

For a team presentation, identify the Mac that will host the deck and connect the demonstration devices there. Keep a simple still slide explaining the same point in case the live demonstration cannot be restored promptly. Before the real session, repeat the source check after reconnecting cables or changing hosts; yesterday’s successful camera preview does not verify today’s selected slide source.

Reference: Apple: live video in Keynote.