Video poster frame flashes at beginning of slide in Keynote

This has been asked before but I haven't found a real solution.

When using videos in your slide show, it's helpful when presenting and working on it to be able to see a thumbnail of the video as a reminder of which slides are coming up. However, that "poster frame" preview shows at the start of that slide. I understand that this is intentional (incase someone actually wants that preview to show), however, I suspect that 99.9% of presenters only want this to show on the presenter side (and don't want their audience to see it).

It's terribly inconvenient to look at blank or black thumbnails when working on a presentation. I can't believe there isn't a built-in solution to see a preview poster frame on the presenter side that is hidden from the audience.

Others have noted a build-out trick using a shape to hide the initial slide, but the instructions aren't clear and I can't get it to work. Also, I suspect that this solution would simply cover the poster frame and thus defeat the purpose of it allowing the video poster to show as a thumbnail. But maybe I'm mistaken and more clear instructions would help.

Here is an example of the hack (from @garyscotland, Movie poster frame flashes at begining of… - Apple Community) which I can't get working:

• place a rectangle shape on a new slide and size to cover the whole of the video image
• add the video to the slide
• set the colour to black or an other colour if appropriate
• add a Build Out using a dissolve or blur effect (Inspector > Animation > Build Out)
• click the build order button and move the shape to the top
• set the shape to with previous build
• set the video start with Build 1