NEVER take photographs of the interior of airports, government buildings, military installations and police headquarters as such photo-ops are almost always prohibited;
When in doubt, always ask a uniformed official in a government facility as to whether photographs can be taken. Otherwise, you may be asked to destroy your electronic media;
If you are a professional photographer always insist in obtaining and completing model releases and offering payment for photographs that may be sold commercially;
Many local citizens have quickly learned that they can earn "easy" money by insisting on being paid. Actually, it is far better to offer payment, thereby enhancing the end-product;
Keep in mind that in many countries that are grappling with domestic or transnational terrorism, photographing police or military can be a crime. Thus, always ask for permission;
Depending on the country, both men and women can strongly object to be being photographed. Always ask for permission;
Parents particularly can strongly object to their children being photographed, particularly where child theft and illegal adoptions are a serious problem; and
Failure to get permission from photographic subjects can often result in seizure of your film, digital media, or camera; calls to police; a severe fine or even incarceration.
The above considered, many nations don't often object to photography by tourists and professionals, yet do be sensitive to the fact that no one enjoys being photographed while depicted in squalor conditions.