You want to get everything right in your novella. If it doesn’t work out, you’ve learned more, and you’ve lost less. Still however, for the very best results you need to flesh out your protagonist.
This may seem strange, given the shorter-length of novellas in comparison to novels. But, with less words, your main character can pack a punch and convey your ideas more succinctly. Now, brain-storming your main muse is easier said than done. But the tip is to focus on one character. This is a novella, and not Middlemarch. You do not need a cast of characters. Check though; make sure your cast list is not too large for such a short story; and contrastingly, make sure your main character cultivated enough.
It’s good to lay out some ground rules when thinking of your character. Like planning your plot, you should plan your character before you begin to write. Here’s three tips to round-out your main lead:
Step one: the basics. Before figuring out their psychological or emotional composition, decide on the fundamental aspects of their persona. Male or female? Where do they come from? What does their hair look like, do they wear glasses, what kind of clothes ? Is their voice soft or harsh; are they melancholic or happy? Deciding on these simple things gives you a plain sketch, so to speak, that you can slowly begin to fill-in.
Step two: Get to know them. Perhaps it sounds silly, but getting to know your main character really works. Think about the quirks to their personality. What do they like and dislike? Do they have odd behaviours? A favourite food or drink? Are they political? Are they a mother or are they purposefully childless? Sit with them, in your mind, and ask them questions - just like a job interview. That’s fun, and it works.
Step three: Pull it together. Try mapping everything into a list, perhaps ideas fit together well into pairs that you hadn’t spotted. If they are from London, does that explain why they prefer the city over country life? If they wear glasses, what does this tell us about their health or occupation? If they are melancholic, can this be linked to their childhood? When you link steps one and two the final picture should start to form; your character will then be three dimensional.