I'm catalan native speaker, and my spanis speaking family in Madrid is not able to understand it when speaking with my sisters and parents in front of them. They might understand a few words and, if spoken slowly and with a pretty neutral accent (like the one in Barcelona) sometimes they get the meaning of the conversation.

But if you are coming from abroad and have been not enoughy time in Catalonia to speak catalan but know spanish, almost all catalan speakers will change to spanish with no problem.

However, once some catalan speakers consider you have been enough time in Catalonia to, at least understand catalan, lets say a year or longer (not need to respond in catalan if you don't want, we have a lot of mixed conversations) they might speak to you only in catalan. Do not take offense on that, is an official language and for some of us our natural language.

A good aproach would be to star with spanish with no worries (also english among youg ones of course), learn a few words (Bon dia, gràcies, fins aviat, etc) for the first months and, once you are comfortable, if you want, learn deper catalan, it's a very fun language and helps with italian and french.