SPOILER ALERT! In this article I'll discuss some of the villains in The Secret of Red Gate Farm and The Mystery at Lilac Inn.
In a Nancy Drew Mystery, it's almost always obvious - often to Nancy as well - who the bad guys are. They're the ones with bad attitudes and they often appear as (in the words of Carson Drew, Nancy's father) "tough customers."
They're also, often the only characters who get much of a description at all. (I'll discuss that in a later article in this series.)
For instance, here is one of the bad guys in The Secret of Red Gate Farm:
He was tall and wiry, with hostile penetrating eyes and harsh features. His suit was bold in pattern and color, and his necktie was gaudy.
...she would not have forgotten such a cruel face. His steel-gray eyes bored straight into her.
Once again, the Mystery at Lilac Inn is the one book that breaks the mold. Since the main villain is impersonating Nancy, she can't be portrayed as ugly. However the moment Nancy learns her name, she also finds out the woman was in prison. At which point the reader says, "Bingo, that's your bad guy."
Beyond that exception, the crooks in these stories are depicted as coarse, slothful, wearing too much makeup, miserly, cruel, obnoxious in their dealings with others. In some of the older books there are also definite issues of racism, which appear to have been edited out, or alt least lightened in later editions. Invariably, however, in some way a Nancy Drew bad guy will appear as a lowlife from the first moment we meet them. Figuring out that they're the fishy ones is a no-brainer.
In your own writing, remember that all the villains don't need to be irritating, ugly, or otherwise slimy. Some of the best bad guys are the ones who appear charming or innocuous until they're revealed.
Even if your story isn't a mystery, remember that most of your villains are "normal people" except that they're in opposition to your protagonist. Give your antagonists dreams, wishes, and strengths as well as flaws. Fill your stories with characters that don't fit a set mold. Your readers will thank you for being unpredictable.
The What I Learned from Nancy Drew Writing Series:
Intro to What I Learned from Nancy Drew
Part 1: Contrived Beginnings
Part 2: Lack of Red Herrings
Part 3: See Through Bad Guys
Part 4: Undescribed Characters
Part 5: Too Many Characters at Once
Part 6: Adverb Abuse
Part 7: Unnecessary Scenes
Nancy Drew Mystery Stories
Nancy Drew Games