What is the difference between an adjective before the noun and after the noun?

For a long time I’m having trouble understanding the difference between two kind of expressions like those below in terms of meaning, not grammar:

Excited people are looking forward to seeing this event.
People excited are looking forward to seeing this event.
EDITED TO USE CLEARER EXAMPLES:

“All navigable rivers are being patrolled.”
“All rivers navigable are being patrolled.”
As a native speaker, how do these expressions in bold sound to you? Is there any difference in meaning between examples #3 and #4? If so what is it, and why is this so?