In the spoken language, verbs "falar" and "dizer" are often interchangeable. There are situations when they are not, though. Is there any rule of thumb about this to help those who are learning portuguese? e.g. In ptBR there isn't much difference, if any, between: fale a verdade, diga a verda...

I think nobody asked about the difference between ser and estar. I know the general rule is that "ser" apply to permanent states, but there are some tricky cases for non natives, such as : ele está morto O teu carro é novo Elas são amigas inseparaveis Eu sou consultor ele é diabético ele ...