Have you ever noticed that whatever group of people you think of, you think of extremists? Think about Muslims, Christians, Italians, Germans. Whatever the group (unless it is one you belong to), you probably think of the most extreme, stereotypical, outlandish examples.
I'm sure it's just the way we are wired. The best way to remember a collective is by its differences. And the best way to remember differences is to drastically overpronounce them. Even still it leads to some dangerous consequences.
For one thing, it makes it very easy to be bigoted. When you start thinking of a group of people based on the worst and most offensive versions, you stat thinking that every member of that group acts as such.
For another thing, it makes for really bad writing. You start getting into the idea of defining people by a single quality, and that all people from that country or in this religion all act the exact same way. Sure makes for boring characters.
Think of people as people. It's advice good both for writing and for life. I won't be so foolhardy as to say that assumptions don't get you far. Stereotypes exist for a reason. But they are also generalities and not laws. Avoid putting your foot in your mouth.