Maybe it's a rookie writer problem, but I've caught myself far too many times attempting to recycle a nervous tic with different characters...sometimes within the same book. Doh! Brow rubbing? Lip biting? Hand on hip? Thumb tapping?
I have to keep a spreadsheet to track who has what tell.
I'm not proud of my limited interpretations of body language, so tracking that stuff is as necessary as keeping track of who has what physical traits. These days, when characters within the same book share a tic, there's a reason for it. ~muuwahaha~
What about character voices? How do I keep all my assorted protagonists from sounding the same, particularly within the same genre? I do what Jeffe suggested, I build full lives for them (with back-stories only I know) and I make them as vastly different from one another as I can. Those differences are reflected in their manner of speech.
As for secondary characters...I don't know that I've been as conscious about making them unique from secondary characters in other books. Mostly because all other characters in a book are a reflection of the environment and protagonist. I'm hoping if I've made those two foundations unique, the rest become unique organically.
I might get called on having "ditto" casts.
Fortunately, Excel has a multi-tab option that I do know how to use. Hopefully, I won't have to resort to...Access Databases. ~faints~
What about you, dear readers, have you read an author's works and noticed they cling to a certain type of characters, mannerisms, or bad habits? Which traits do you find to be overly common?