''In some ways, it is. I make fewer mistakes now, technically. Let me think for a moment. Well . . . I know when a new character can be introduced without a lot of background detail. And I know when a chapter can be hurried along. Generally, I've acquired a better sense of pace.'' Another question on technique: As between plot and character, which way does Mr. Yates lean in his novels? ''I would say that the balance now is more toward plotting than when I was younger.'' Unlike most authors, Mr. Yates writes fiction full time because of an unusual arrangement - he receives a monthly check from his publisher.''I'm grateful to Seymour Lawrence for setting it up while he was affiliated with Delacorte Press. All I'm expected to do is bring in the book on time. The new one took nearly three years. Sometimes I do have to stop to teach - I did at Boston U. during this book - but otherwise I'm a self-employed novelist.'' Although Mr. Yates has been living in Boston for several years, the West Village in New York is more like home. Why Boston? ''It's relatively quiet here. The atmosphere is peaceful for a writer.''