What qualities separate those few who acquire native speaker fluency, from those who are only fluent?

Question

Assumptions

1. Abbreviate Native Speaker Fluency: NSF.

2. NSF excludes L2 learners' noticeable, but insignificantly different, accent because native speakers can understand them. But NSF necessitates mastery of grammar, unlike fluency which allows errors of form inasmuch as native speakers still understand the content.

3. Acquiring NSF becomes extremely difficult (but NOT impossible) after adolescence (12-18 years old).

4. I consider only NSF L2 speakers who learned a L2 after adolescence, and not those who were somewhat exposed to it before (e.g. Nabokov, Stoppard).

Question

As most adult L2 learners fail to acquire NSF (per 4), what factors or traits enable or help those who acquire NSF and overcome the common barriers?

Some examples: Joseph Conrad, Tadashi Tokieda (L1: Japanese. L2: French. He also studied many other languages including Latin and Classical Greek.)

Answer 1

Since it is difficult to find literature about this, I'll focus on a "case study". One of the most famous foreigners in China is 大山/Dàshān/Mark Henry Rowswell, a Canadian whose fluency in Chinese is so high he can even do stand-up comedy (相声/xiàngshēng).

Dashan started studying Chinese at the age of 19, namely at the University of Toronto. There are a few clues about how he learnt Chinese in an interview on Chinese Talkeze and in an article on Zhengzhou Connections.

1. Dashan enjoyed learning Chinese.
2. Dashan says his success is not based on a special gift for language learning.
3. According to comments on Sinosplice, the Chinese-speaking populaton in Toronto speaks mainly Cantonese, which is not helpful if you are learning Standard Chinese.
4. During his third year at university, his Chinese teacher started introducing authentic materials:

   Anyway, it’s very difficult at first, but I think the biggest advantage is that you can learn from authentic materials that way. Textbooks are written by Chinese people for foreigners, while novels or newspapers are things that Chinese people write for themselves. So the same thing, we [English speakers] will make textbooks for Chinese people probably differently from the way we would actually speak to native speakers. So I think when you get to a certain level, it’s good to get away from the textbooks and just go to the real living language that people actually speak. (See Zhengzhou Connections.)

5. You need to choose your learning materials very carefully; they should be neither too difficult, nor too easy.
6. "Dashan is sceptical to just settling on any specific technique." (See Zhengzhou Connections.)
7. You need to use the target language in your daily life. Treat it as a "second language" instead of a "foreign language". (This reminds me of the advice about using your computer devices in the target language on the website All Japanese All The Time.)
8. After he got his BA in Chinese Studies from the University of Toronto, he went to Bejing University. He also fully immersed himself into the language, i.e. he moved into an environment where people only spoke his target language and no English.
9. "Dashan worked on his accent just by being immersed in the environment." (Interview on Chinese Talkeze.)

From the list above, the things that "average" learners are least likely to do, and which may therefore separate Dashan from other learners, are the following:

1. Using authentic materials as soon as that is feasible.
2. Integrating the language into your daily life instead of treating it as something "foreign".
3. Immerse yourself into the language, i.e. move into an environment where only your target language is spoken.

Something that does not separate him from other language learners is focusing on specific techniques.