Characters can always speak through "their" writing - diaries, letters to others and so on. Characters can also speak in the way that they talk. For example, L'Evallier, the Queen's Chief Elf, will never use contractions and as a result his speech is far more formal than even his boss's. Even the Queen will occasionally use an apostrophe, he will not!
Characters can also speak through how they treat each other. Eileen treats Brankaresh with contempt (he returns that compliment!) but generally treats others with politeness and, where she feels it is merited, respect. Eileen is considered to be the snob's snob as a result! Hanastrew is down to earth with all and is more liked as a result.
So what are your characters telling you and your readers? And are you aware of what your characters are saying or do they surprise you (sometimes)? One of my favourite things as a writer is when I write a line and just know it is exactly what that character would say, but I also relish the "out of the ball park" moments when my characters surprise me. I like to explore where that takes me as I nearly always discover something more about my characters.