He becomes more vulnerable. Usually, Carlisle is seen as this strong, composed leader. But when he falls in love with Bella, his emotions start to get the better of him at times. He might second - guess his decisions more often.
Often, it starts with an unexpected encounter or a moment of realization. Maybe Carlisle sees Bella in a new light, not just as Edward's girlfriend but as an individual with her own charm. Then, there are small gestures of kindness and understanding between them that gradually build up the relationship.
In some fanfictions, their relationship might start with Carlisle being drawn to Bella's unique spirit. He, as an older and more experienced vampire, is intrigued by her resilience. Maybe he first notices her when she enters the Cullen family's orbit, and from there, a slow - burning attraction begins to form as they interact more and get to know each other on a deeper level.
One common theme is the exploration of forbidden love. Since Carlisle is an older vampire and Bella is a human (initially), their relationship goes against the norms of both the vampire and human worlds.
In the fanfiction, Carlisle's sense of duty to his family and the vampire code might be a hurdle. He has to balance his feelings for Bella with his responsibilities to the Cullen family. Also, Bella's own human life and her relationships with other humans, like her father, could be a complication. How can she maintain those relationships while being in love with Carlisle? There's also the issue of secrecy. Their relationship has to be kept hidden from the outside world to protect both of them.
One interesting story could be where Carlisle's love for Bella is a slow - burning one. He tries to resist it at first because of the complications it might bring with Edward and the rest of the family. But as time goes on, he realizes he can't help his feelings.
One popular 'bella falls in love with carlisle fanfiction' is 'A Different Kind of Love'. It explores a unique take on their relationship, with Bella being drawn to Carlisle's wisdom and kindness in a more profound way than in the original story.
At first, it would be a bit awkward as Bella adjusts to her new life. But Carlisle would be very patient and understanding, like a father figure. Esme would be more of a motherly presence, always there to comfort Bella. As time goes on, they would become closer. Carlisle might teach Bella about the vampire history and ethics, and Esme would share her love for the family and home. Eventually, they would have a very strong, loving bond.
If Bella was Carlisle's daughter, her relationship with the other Cullen siblings would be altered. She would be part of the family in a more fundamental way. For example, Alice might see her more as a sister to protect and guide, rather than just a friend. And Rosalie might have a different attitude towards her, perhaps being more accepting since she is Carlisle's daughter. Jasper could also be more involved in her training or development as a vampire, as he has his own skills and experiences to share.
The Cullen family members are important characters too. Edward, for example, might have an interesting dynamic with Bella in this fanfiction. He could be either protective or jealous of Carlisle's special relationship with her.
He might be very empathetic. He could offer her a safe space and listen intently. Maybe he'll use his calm and understanding nature to make Bella feel supported.