Normally, when right clicking on an upper face of a block with a placeable block in your hand, that block gets created in front of you, and one block in the stack you are holding disappears This still happens if you click on an upper face that is not at the top of a block, such as right clicking on top of a slab, or on the base of a cauldron.

Example

However, when there is water in the cauldron, you can no longer right click on the bottom of a cauldron, and place a block on top of it. This is inconsistant with normal water, in which you can place blocks through.