The last on the "don't do" list that Mark Twain came up with is this. "The characters in tale be so clearly defined that the reader can tell beforehand what each will do in a given emergency." This in a way ties back to the characters reason for being, which I covered back in chapter six of this essay collection.
One of the elements ties into the fact that a character should be able to do something in a given emergency. I'm not per say talking about something positive either. They can run around like a chicken with their head cut off and they would still be doing something during the given emergency. But a character shouldn't be there doing nothing at all. Even if they observe what is going on, they are doing something.
However, the main element comes from the fact that each character should have their specific task that they are able to preform. A writer shouldn't pull out some random skill for a character at the last minute simply because they can't think of any other way out. This is a form of deus ex machina, and the manner in which this trope is used isn't the positive manner.
The one exception to the rule is if the ability is something that a character has been hiding for a given reason. For example, their family may have been hunted down for such an ability. Or they think their manhood is challenged by doing something as sissy as healing. This instead plays a part in character and plot development. This is particularly true with a mystery series.
One thing you want to be careful with is the all around genius. While it is nice that some characters can do anything and everything, they will still have some sort of fatal flaw to them. For example, they can easily get bored or they have some sort of social problem. Other times they could be good at something, but they find being good at said something is just plain boring so they don't bother learning said task.