Memoir is by definition “an historical account or biography written from personal knowledge or credible resources.” Most memoirs I have read do contain descriptions and or conversations that may not be exactly as they happened. This seems to be generally accepted as long as the essence of truth is obvious. However, it is completely unacceptable to lie, to make up stories and suggest they are historical. Don’t do it. And don’t think that creative nonfiction allows for it.
Creative nonfiction allows for an element of creativity in our memories, an element that sometimes rearranges what is recalled in order to convey the truth. Without truth, memory does not really exist. However, sometimes in order to provide a feeling for the times or an understanding of all the complex forces that may have influenced behaviors, descriptions are created, but not events. If description is used or imagined conversation that helps convey the truth, be sure to call your work ‘creative nonfiction.’