amigo is also a token foreign word, used in movies when the script writer wants you to really understand that the action takes place in Spain,Mexico or some other Spanish-speaking country.
Much like Garçon in French, it is nowdays very rarely used in real life to address people.
For example, if in Mexico someone hails you with ¿Amigo, quiere un taxi? you can bet that he has taken you for a foreigner; the common term would be señor / señorita, señora (or caballero / dama in a very formal context).
In Mexico, you should follow the same practice; señorita is almost always appropriate. With friends, you can say mano, gúey or even cabròn - this last one is a swear word, and you better be sure that the person will not take offence!
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