You can also develop their characters through their interactions with other characters in the fanfiction. If there are friends or family members involved, how Bella and Will react to their opinions and support (or lack thereof) can say a lot about who they are. For instance, if Bella's family is against the pregnancy, does she stand up to them? And how does Will support her in that situation? This can all help to flesh out their characters more fully.
For Bella, you could show her growing in confidence as she deals with the pregnancy. At first, she might be scared and unsure, but as time goes on, she could become more assertive in making decisions for herself and her unborn child. For example, she could start taking charge of her prenatal care and making plans for the future, which shows her strength.
Will could be developed as a character who is initially in denial about the pregnancy. He might be focused on his own goals and dreams, and the pregnancy throws a wrench in those plans. But then, as he sees Bella's determination and starts to feel the baby move, he could gradually become more involved and protective. This transformation in his character would be interesting to explore.