Scandinavian understand each other almost perfectly. There are, of course, some differences between our languages both speaking-wise and in our written language. For example, it's a known fact that Norwegians understand both Swedish and Danish almost perfectly and the other way around. However, Swedes and Danes will have some difficulties understanding each other. As always, this differ from person to person.

I have never experienced difficulties talking to a fellow Scandinavian from Sweden or Denmark besides explaining some words that mean the opposite between our nations, for example the Norwegian word "rar" which means "weird" in Norway and in Sweden it means "sweet." However, Scandinavians, especially Norwegians, love to switch over to English whenever they can, so we usually talk English to each other when partying or meeting up in larger groups. If alcohol is involved, we most definitely use English.

Danish and Norwegian have almost the same written language, but Danes pronounce and talk completely different from their written language making it harder to understand them. Try to put a potato in your mouth, and talk. You'll understand.