To my mind, a character who routinely muttered and mumbled would seem eccentric, to say the least, if not sullen or belligerent. An image that comes to mind is an old estate caretaker, wiry and bent with age, given to conversing with plants as he prunes them. Another is a street person suffering some form of mental illness.
Not long ago I read an adventure novel, which charity demands I refrain from naming. I’ll refer to the author simply as Jane. The two main characters were teenagers of extraordinary intelligence and achievement. The imaginative plot held my interest, but as I read, I quickly became distracted by the number of times these brilliant, well-behaved young people muttered and mumbled. Hardly a page went by without one or another of these words appearing in at least one dialogue tag.
Maybe I would have been less aware of this if I didn’t teach classes in Writing Dynamic Dialogue, but having fairly recently completed an in-depth study of that topic, I took a closer look. I noted that on rare occasions, muttering did seem justified, but each of these circumstances would have been better served by simple whispering, a tag the author rarely used.
I soon realized that Jane had used muttering and mumbling in situations more suited for internal monologue. She did use internal monologue on rare occasions, but not nearly as often as it was called for. I commend Jane for using dialogue rather than expressing the characters’ thoughts as straight narrative. Obviously she took lessons about “showing rather than telling” seriously. Dialogue is a powerful way of “showing,” but not if it results in distracting behavior not in keeping with your character.
Perhaps she heard or read a caution somewhere about limiting use of internal monologue. Although I have never come across such advice, a liberal mixture of internal monologue and standard dialogue could be confusing, with the use of quotation marks seeming almost random, and a manuscript using italics for extensive monologue may look cluttered to some.
In my search for a cure, I found a number of passages where creative editing could allow the main characters to show reactions and perceptions non-verbally rather than using either dialogue or internal monologue. This book was fiction, but memoir writers face the same challenge — to reach beyond the obvious and find innovative ways around the speed bumps that distract readers and cause them to put our volumes aside.
Write now: do some writing practice, filling a page with snippets of random self-talk or internal monologue. Don’t make it one long string — use short pieces, not longer than one line each. When you have half a dozen examples, explore non-verbal ways of demonstrating these thoughts and reactions.