Why does Donald Sutherland know Italian sometimes, but sometimes not?

He knows enough to get by but is not fluent.

I was wondering that too. At times he talked like he was fluent and other times he looked like he was hearing Chinese. His performance was Abysmal. Not remotely convincing as an architect/archeologist or whatever his title was.

When you're learning a language, you can get pretty good at some of it, with words that you use every day and people that you're at ease talking with, but still stumble with other parts of it when you come to an unfamiliar situation and don't know how to phrase what you want to say. The fact that he was agitated and stressed could also have played a role.

I read something someone once said about a second language they'd learned, they were a diplomat or a businessperson and they needed to negotiate some issue with representatives of a foreign company or government, and they were able to get by with discussing the particular thing they needed to talk about, but were surprised to find themselves having difficulty with basic everyday life conversation when they tried to engage in it.

I've found myself in similar situations too, where I'm able to talk about one thing with relative ease but find myself completely at a loss for how to say something when I'm in unfamiliar territory, discussing something I don't know the words for.

I imagine his Italian is poor and he knows some common phrases he needs (General greetings, common phrases for waiters/drivers and some work related words/instructions to aid his work with his Italian speaking underlings). But when having to explain something in depth on the spur of the moment regarding a topic he hasn't researched in Italian, his poor knowledge of the language shows.