"Sir" is the polite form of address if you don't know a person. For example, if you see a man that you don't know drop his wallet in a parking lot, you'd call out "Sir!" to get his attention. Unless you're saying "Mister Smith" or whomever, then saying "mister" is the same thing as saying "fellow" or "guy" (much like you've seen on police TV shows).
In the US, it is customary to address a new acquaintance as either Mister or Miss Lastname until they tell you, "call me John." In the South, it is traditional to call one's elders by the honorific and their first name, such as "Mr. John" or "Miss Sarah." For example, even though you've known your neighbor, Sam Brown, since you were a child, because he is older than you, you would never address him as "Sam." You would call him "Mr. Sam." Again, this seems to be a tradition exclusive to the southern part of the US; much of the rest of our country is more informal.
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