Sometimes third person novels are still written only from one person's point of view. Many YA novels take this approach. The POV character is usually the 'main' character in the novel. They are the one doing something or having something happen to them. And usually, we think of them as the 'good guy.'
But if you're just plotting out your novel and perhaps filling in that character profile sheet, maybe you're unsure of who your main character or characters are. How do you decide? You might not know until you've plotted out some of your novel. But here's a short test.
Who is suffering the most? Which character is in the most pain? That suffering will draw the reader's sympathy and interest as they hope for and watch this character take action. A character in pain wants to enact change. This character also needs to be in a position to act and have the power to do so. They have the freedom to try and change their situation. Don't make the mistake of making your main character too powerful.
Even in medieval fantasy, unless it's a very small kingdom or the castle is under siege, the king won't be actually be wielding a sword. The captain of a large space ship with hundreds of people serving under him, will not go down to explore a mystery on a strange planet as Captain Kirk did so often. The chief of police in a big city isn't going out on routine calls.
Do you have other ideas of how to decide which character in your story should be the 'main' character? Have you ever changed your mind after which character will play the lead? Do you agree that the main character should be the one suffering the most?
(I might be late responding to comments as I'm spending most of the day driving home from Boston)
Susan Gourley writes epic fantasy and is multi-published in science fiction romance writing as Susan Kelley. Find her at Susan Says.