Manuscript Preparation, Copyediting, and Proofreading

Q. I’m editing a novel in which a character stutters on the first word of a sentence. Are both instances capitalized (e.g., “M-My name is” or “M-my name is”)? And do I use a hyphen or an em dash?

A. We prefer lowercase, though either one is acceptable; use a hyphen rather than a dash. See CMOS 12.44.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]