It is important to emphasize that the government of the DPRK - in particular the leaders Kim Il-sung, Kim Jong-il and Kim Jong-un - are, publicly, very highly revered in North Korean culture. While slavish devotion is not expected from tourists, especially given that the Juche philosophy of the DPRK is specifically aimed at the Korean people only and is not applicable to foreigners, criticising them in any way is highly offensive and illegal, and will get you and particularly your guides into trouble. It is not worth inadvertently threatening their lives by insulting their leaders.
When in North Korea, it is advisable to refer to the country as the DPRK instead when discussing it with your guides.
DPRK stands for the Democratic People's Republic of Korea and is the official name for the country reflecting their belief that the south is occupied territory. Despite what the rest of the world may think, this is what they will refer to their country as. You will also notice this referenced in their literature in the same way (South Korea).
The DPRK has very strict laws about taking pictures though there are many great photographing opportunities around the country, particularly in cities such as Pyongyang. Again, this largely depends on the guides assigned to you and how relaxed they feel to trust that you won't do anything to embarrass them.
While it may have been true in the past to "not look at" or "take pictures of" people in the DPRK, you may be also surprised to be able to take a picture of a wedding couple or of a grandmother taking their grandson out for a walk waving back.
Do not take photographs of anything that could be of strategic importance (i.e. places with a soldier(s)/policemen in front of it) or of things that you been told specifically not to. Again, as emphasized before, always ask your guides if you are ever in doubt.
Bringing gifts such as cigarettes or Scotch whisky for the men, both guides and the driver, and chocolate or skin cream for female guides is a nice gesture. Please be respectful toward your guides, especially since North Korean guides are known to occasionally take tourists whom they trust well enough to see other places and events in North Korea that they wouldn't ordinarily go to.
This can also extend to how freely they may feel about your picture taking. Remember, they may be as curious about you as you are about them.
Most, if not all, tour groups to the DPRK are asked to solemnly bow and lay flowers on one or two occasions in front of statues of Kim Il Sung when visiting monuments of national importance. If you're not prepared to do this customary gesture, do not even try to enter North Korea.
Be mindfil of North Korean culture and be sure you always act in a respectful manner around images of the two leaders.
This includes taking respectful photos of any image of them. When photographing statues, especially Mansudae, be sure to get the entire statue in the photo. Formal dress is also expected at important monuments such as Mansudae or in visiting the Kumsusang Memorial Palace.
Any trouble you cause as a tourist will likely be blamed on your tour guide's inability to control you, and he or she will bear the brunt of the penalties. Additionally, future tourists will be allowed less freedom and will face increased restriction on where they can visit and what they can photograph.
Other than your tour guide, you will likely not meet anyone else in your trip who speaks English; a few Korean words and phrases are a nice internationalist gesture.
Despite the sharp political differences, North and South Koreans generally share a common culture; the various tips in the South Korea article under respect (such as using two hands to pour drinks) will also help here.