I have roughly 30 speaking characters; about ten speak often enough that their voices should be well defined. I have compiled, from web sources, various considerations when building distinct character voice.
Originally, they all sounded similar to each other and to my education and background. I didn't worry about this in the mad frenzy to finish the first draft. Since then, I have been using a brute force, somewhat mechanical approach to making the characters distinct from one another (and from me).
The considerations I use for voice include: Gender, age, education, accents, personal verbal quirks, tone, emotional state, and so on. (I have not really included actions as part of voice, but I think their actions are distinct from one another.)
Thanks to Word, it is easy to go through a 200 page document and find all occurrences of an individual speaking, or of a particular dialect, or word. So I can make certain that Jane's lines are well developed (assuming she is educated) and Jill's lines are not (if she is not educated.) I can also identify my own verbal habits (I used Oh far more often than I realized), and scan through for them and correct them. This is one nice feature of writing with a word processor.
I've assigned specific (largely invisible) words to each character as part of their unique voices. One character is allowed to say Oh, but the others rarely are allowed to say it. Words like Well, to start a sentence. Or Hmm, in response to not knowing something. Or the use of another person's name, as a single word ... command? that that person should behave.
None of these are used overly much, but each pattern is limited to a single person, and the assignment is meant to fit their personality. The character who is most like a scientist, for example, is allowed to use qualifiers like a bit of (e.g. 'It will take a bit of time') but this is limited to him.
I am curious if there is a downside to assigning verbal tags to characters. It seems to me like one fast and effective way to distinguish character voice. As another example: One of my characters is withdrawn and uncertain of herself. She says I think a lot, and this is because she feels she does not "know" things the way others do. I went through the manuscript and made certain that every occurrence (barring a few) of the phrase I think was coming from this girl. I've done the same with verbal quirks for other characters.
Question: Does this approach, which is part of my attempt to distinguish character voices, seem valid? Is there a red flag here? I understand that I don't want to limit myself by prohibiting other characters from ever saying the phrase I think. (In some cases, where it was appropriate for them to say it, I left it. In others, I changed it to something like If you ask me.) Aside from the possible limitation of this approach (using a sort of mechanical approach instead of a more organic writing approach), are there pitfalls you can think of, that I am not aware of? Or, perhaps there are other simple strategies akin to this, but different, that you have used to distinguish voice.
(Thank you for the comments about actions to emphasize voice. This is something I hadn't put together before.)