Answer
The thoroughness of an inspection cannot be predicted from a single trip. At one crossing, officials may limit themselves to documents and questions; at another, they may ask you to open the trunk or direct the vehicle for additional inspection. Border and customs authorities may inspect the vehicle, passengers, luggage, and the grounds on which goods are imported.
The driver needs valid vehicle documents, a driver's licence, insurance, and the owner's permission if the vehicle belongs to someone else. Each passenger must meet the immigration requirements. Food, plants, animals, large amounts of cash, and commercial goods are regulated separately and may need to be declared.
Don't plan on the assumption that inspections are usually lax. Before travelling, check the rules of Aduanas and the immigration service, organize your documents so they can be presented quickly, and answer the officer's questions honestly.