Excellent question. I should ask one of my characters to answer it for me. 🙂 If we define smarter as have more knowledge and being able to access that knowledge and apply it to a new situation, then it’s fairly easy.
We have access to knowledge as authors. I did a week’s worth of research on Mars’ atmosphere, terrain, and orbits for the Black Hole Sun books, and for a book that I just finished, one of the characters knew everything that a geologist would, and she used that knowledge to save others. In real life, I don’t know any of that.
Letting the character use that knowledge is easy. We create a plot and events where they have show off for the reader.
The difficult part is make a character more wise and more insightful than we are as authors. That is much more difficult because it takes steeping back from the story and listening to both your characters and your expert readers (in my case, the best editor in the world). In these cases, we can only hope that the wisdom and insight find their way to the page.