[jaclaz]

Dialects are rarely written, they are spoken and while (obviously) usually someone natively speaking a given dialect can talk with someone from another near region, dialects from the north (say Lombardia or Veneto) are so different from those of the south (say Campania or Sicilia) that they are effectively distinct languages.

The above is the hypothetical case of two unschooled people, as said before most if not all italians are bi-lingual with their own dialect and switch automatically to italian (possibly with some local peculiar forms or accent) that is the common language when talking to some "foreigner".

As you say, italians can usually understand (in written form) a dialect because of the common roots, but for what it matters an Italian with some good Latin knowledge can usually get the overall sense of written text in Spanish, Portuguese, Romanian and possibly French.

The unification of language to Italian has been a process that went on for decades, through schooling and later radio and television, when Italy was re-united in the decade 1860-1870 it was a tower of Babel.

Imagine the conversation between a (unschooled) soldier from Friuli attempting to interact with a (as well unschooled) farmer in Sicily in the early years of the unified state.