Just as the title says, you place blocks on top of a cauldron when standing right inside it by rightclicking on the top of the cauldron you're in
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Confirmed, also @Meta Kumasasa makes a valid point as well.you shouldn't be able to place a block if you're standing on it either. I also noticed if you place a cauldron 2 blocks under ground, then place a glowstone block on it while you're standing on the rim of the cauldron, then fall into the cauldron, you can see the through the ground.
Confirmed.
For any non transparent block, the player will be pushed off the cauldron.