Like everywhere, immigration controls have the goal to make sure they let in people with legitimate reasons (tourism, business, visiting friends or family...) while keeping out people who are actually trying to immigrate (initially pretending they're just there for tourism, but planning to remain in the country, stay under the radar, work illegally, etc.).
This actually applies whether you need/have a visa or not. When you have a visa, most of those checks are done in advance (though at the border they may check for consistency). When you don't, this has to be checked when you enter then country (or the Schengen Area, in this case).
In the vast majority of cases, they will ask a few questions. They may be as innocent as "how long are you staying for", "what's the purpose of your visit" or "where are you staying", and then off you go. They may be a lot more detailed if the passport control officer has any doubt regarding the intentions of the traveler.
The traveler should have a return ticket they can show on demand.
They should have money or means of payment that allow them to sustain themselves, consistent with where they intend to stay. If they are staying with friends or family, less is expected than if you are staying in hotels. It doesn't need to be cash, credit or debit cards may be appropriate. But if they do have doubts, they can indeed ask for more details, like showing a balance or a credit limit that is consistent. If you are saying you are staying with someone, they may want to call them if they have any doubts.
The real keywords here are consistency and credibility. If your story is consistent and credible, it should be real easy and quick.