I know a lot of you who play this game. Start a quote, and have your friend finish it, or take the next part. Quotes from The Princess Bride. Or from the Hitchhiker's Guide to the Galaxy. This week I was playing the game with my son and daughter, quoting from Rise of the Guardians. Sometimes it only takes one word ("inconceivable!") or maybe a couple of words to bring a whole quote or sequence of words to mind.
It's not just entertainment stuff, though. We have quotes from the internet, but also quotes from the deeper books we've read, or the Bible, or just sayings that have been in the family for years. The frequency with which we have whole chunks of language float up in our consciousness is quite remarkable.
In the study of language and language learning, people have discovered that a lot of what we learn is based on phrases that come into our repertoire in chunks, rather than single words that get put together using rules. In America we often like to vary those phrases in (usually predictable) various ways. In Japan there are a lot of set phrases that must be delivered as is, in the required situations (and variation is discouraged).
What I'm getting to here is an idea about how to make the inner voice of a character come alive. How to get the character's mind to demonstrate the culture he or she is a part of, and how to weave backstory into the character him or herself.
Think about what your character has been exposed to. What kind of colloquial sayings? What kind of family sayings? What kind of books? What kind of other media? All of those things will be folded into the way that character thinks. A character who has spent a lot of time using the internet may use internet slang, but will also likely be inclined to drop themselves from the position of sentence subject, because that is one of the major grammatical features of internet talk (due to the presence of an identity marker on each post).
For each feature of the backstory, or feature of the culture, or feature of literacy and education in your world, try to think of a way that feature would be expressed in your character's internalization. Your character might be a direct quoter, as Janice Hardy's Nya was, always quoting her Grannyma. Or he might be someone like Herbert's Gurney Halleck, who quotes from the Orange Catholic Bible. Your character might also be someone whose language patterns are influenced by a particular cultural tradition or set of metaphors without actually involving direct quotes.
Regardless of what the different contributions have been to the internal voice of your character, it can be very helpful to think of a character's mentality as a symphony of different voices. The character him or herself becomes the conductor, deciding when each voice is most relevant. Letting aspects of different philosophies come to the fore as they become relevant, and choosing which of them to express in the judgments he or she makes. If you can make this happen in a character of yours, then you can achieve a sense of complexity and depth in many fewer words than you otherwise would.
It's something to think about.