Have you ever been in a situation that you wanted to sell an image, but were unsure if you could, because there were people in it and houses? If you had liked to sell it to Getty Images, they would have told you immediately that you would need release forms to do so. A release form for each person, also miners and for each house would be needed.
Do you have release forms right at your disposal whenever you walk the streets? Probably not and even finding out where to get them can be a mission. At least it could be a mission, but there is help. Getty Images itself provides release forms to be used by photographer who want to submit images to them. And there is also Docracy, a website that provides all relevant forms for photographers, put together by professional photographers in the United States. Even contracts when you want to work with galleries and other really important forms are available on their website to make your life easier and safe with regards to your rights and a proper payment.
You might want to consult an attorney anyway, but the forms make you already wiser for the conversation with the legal professionals.
And now the probably most difficult part of having the forms with you whenever you are out to snap away. You will really not find that person ever again you just snapped at the park, just let the person sign the form and you are safe when selling the image.