There are hundreds of recipes on the internet instructing on how to make liquid soap by "melting" a bar of soap - castile or any other. And although it might seem a great innocent idea, the potential for bacterial contamination is great.
To better understand why liquid and solid soaps are different we need to learn on how true soaps are made. Solid soaps and liquid soaps although similar because they both foam and clean, do have their very well established differences and unique processes. Solid soaps are made with a chemical called Sodium Hydroxide, commonly known as lye, while liquid soaps are made with a chemical called Potassium Hydroxide, surprisingly also called lye. These two different quite harmful chemicals (alkalies) when mixed in the right proportions with the oils, produce a reaction known as saponification. For more information on how true soaps are made please visit "Is it a soap or a detergent bar?".
When the saponification of fats and sodium hydroxide occur, the end result is a solid mass with very unique characteristics and a specific pH. Solid soaps aside from being always cloudy don't require any type of preservative due to their high pH (between 9-10).
When the saponification of fats and potassium hydroxide occur, the end result is well, a blob that still needs to be transformed into liquid soap. This blob is mixed with enough water and other ingredients to render it liquid enough to be poured, squirted or pumped out. They also require a preservative since water drops the natural alkaline pH of this soap, making it the proper ground for bacteria and mould to grow if no preservative is added.
When a solid soap is transformed into liquid soap by boiling it with water, the end result is a cloudy gooey mass, so lots of water (much more than to manufacture real liquid soaps) is required to make it liquid enough, at the same time dropping its pH to unsafe levels. The new liquid soap not only is never clear or transparent like properly made liquid soaps, it also grows bacteria and mould, and surprisingly doesn't clean as much due to it's over diluted state. Because of its properties, liquid soap made from solid soap has the tendency to regroup into a solid mass, rendering a soap with a "snot" like consistency, which can be quite unpleasant.
It's better to stick to properly manufactured liquid soaps instead of putting your family and friends at risk, but if you are still planning to make liquid soap from a solid one, there are a few things you'll need to consider. 1) Always use purified water instead of tap water, if you don't have purified water, make sure to filter your tap water with a proper filter and boil it for 10-15 minutes to eliminate all bacteria, and 2) always use a good preservative.