I watched "Jason and the Argonauts" maybe 100 times as a kid.
This was the early days of VHS. My sister would watch "The Outsiders", I'd watch this.
There was a time I could recite the whole film from memory. Memory today, not so good.
The sword pops out of the ground.
Close on Jason.
The camera is also moving in on his face here. We all know it's a pain in the ass to move the camera in stop motion. It's an important element of film, though, especially a fight scene (and we're about to witness one of the great fight sequences).
Here we take the simple way, and add the dynamic by moving on the live.
Then a cut back, the skeleton is coming up (motion is in the animation, not the camera).
And here's the classic "sell". Back to the wideshot with the rear projection.
This is as foolproof and textbook as you can get to sell a composite in the simplest fashion.
Then back to Jason, live action, reacting.
The intercutting of live with animation is as important to making everything feel in the same space as a matte-free composite.
The actors must appear as though they're in the same space as the animation, as though they're seeing it as we are. If that's established the important step of the audience occupying the same space as them both is a bit smaller.
More in a few days...