Assuming all your papers are in order and you are not drunk you have nothing to fear. The Police (GNR) are usually very polite and often speak English and or French. What they are normally looking for is to see that your car is registered in Portugal, you’re driving licence and passport have the same name (always carry your driving licence and passport when driving) and that your car is taxed, tested and insured. They may have a cursory glance around the car to see if the tyres are bald or that there is anything hanging off the car.
If they ask you to open the car they are looking for contraband. Items which have been imported from other countries without paying Iva (vat). They may also ask to see your warning triangle and hi visibility vest which are legal requirements here.
While you are looking through your cars documentation the police officer will stand back from you a little way. Do not hand him your car information wallet and ask him to look for the papers he wants. It can look like bribery and that’s the reason he is standing away from you in the first place. During one of these document checks if you smell of alcohol you will be breathalysed. The rules are the same as the UK for alcohol levels – so beware.