What is the origin of the idiom "on the spot" and when to use it?

The origin of the idiom "on the spot" can be traced back to the literal notion of being physically present at a particular location where an event or situation is occurring. The phrase metaphorically signifies being in the immediate vicinity or in close proximity to where something is happening. It is used to imply being in a situation where one is expected to take action, make a decision, or respond promptly.