Being swedish (but this applies to many other countries as well, read more here), we are eligible for a visa-on-arrival when flying in to Brazil. This visa is valid for at most 90 days out of a period of 180 days. We booked flight tickets to and from Rio de Janeiro with 144 days between arrival and departure. As can be seen on our travel plan we will not be staying in Brazil for the whole duration but intend to more or less explore a large part of south america during this time. We plan on traveling by bus and not fly anything between the beginning and end of the journey.
As we dont like to come to the immigration office without knowing what rules apply and not we searched the web. This gave a very mixed result. The main concern seems to be that leaving Brazil by bus is not a legit explanation that you will not overstay your visa. Some people told stories about not being able to board because the airline could not verify that they would leave Brazil between the arrival and departure. Some made fake e-tickets to prove that they would fly out of Brazil and into Argentina for example, as flight tickets are supposed to be accepted as proof that they will indeed leave the country. A third person bought flight tickets that they did not use.
Reading all this (the day before departure at 9 in the evening), we concluded that we will buy flight tickets from Sao Paulo to Buenos Aires and not use them. Said and done, we were now out €150 each.
Continuing on to our experience of what happened. We flew from Stockholm, Arlanda to London, Heathrow and then on to Rio de Janeiro, Galeao. When checking in and boarding at Arlanda, we were never asked about anything besides our boarding pass (which we hade printed at home) and our passports. After we arrived at Heathrow I thought that this is it: now I need to be prepared to explain myself that I do not plan to stay in Brazil forever. Nothing. The lady at the transfer desk took my boarding pass, scanned it, and let me through.
Okay, so we bought tickets we wont use, and not a single person wants to see them. On the almost 11 hour long flight we had good time to think about what the immigration in Rio might be like. We got our immigration cards and filled them in, nothing out of the ordinary there.
Arriving in Rio and walking through immigration also turned out to be a rather simple process. We didnt even have to walk through it, you could walk around it, like everyone who didnt need a visa did. The person that you met there didnt care much for your documents: they scanned your passport and stamped it together with the immigration card.
The conclusion is that British Airways and the Brazilian immigration police were not interested in seeing any proof that we are in fact going to leave Brazil during out trip. We bought tickets that are now worthless, but on the other hand saved us from having to worry about this during the flights.