In your world, your characters are established as very real living, breathing beings. This means that they should have complex histories and personalities. It’s very important that you keep that in mind and that you remain true to that as you write.
What do I mean by remaining true to them?
We’ll say that Sally is an introverted book lover who’s never been kissed. If Sally were to suddenly throw herself at Toby, that wouldn’t be true to her character. It wouldn’t fit in with her introverted nature, and it breaks away from the nervousness associated with having never had intimacy with another person before. That break from her established persona and known history will confuse the reader and break immersion; as we all know, you don’t want to break immersion.
This relates back to the character development, where I stated that you need to know everything about your character and how that impacts them. Now, you can use steps away from the established persona to hint at something that currently remains unknown to the reader. For example, you could have the usually hard-faced and cool Zach break from that mould and risk being late for his meeting with his superior to save a fluffy chocolate-coloured puppy. That breaks away from his usual obsession with promptness and strict adherence to guidelines. The reason he did that was because he had a very similar puppy through his childhood that got him through the emotional abuse he suffered at the hands of his mother. That puppy represents the basis that he built his stern, cool persona on and the foundations for what he perceives to be his strength. Therefore, he breaks from that to save the puppy due to the huge emotional and psychological ties back to the puppy of his childhood.
The reader doesn’t know that; all they know is that he chose to save the adorable puppy. That does, however, give a hint at something else within Zach and open up the writer’s opportunities to expand upon Zach’s past and add depth and history to his character and persona. The writer will need to expand upon that and hint at Zach’s reasons and past during the puppy saving scene. It can’t stand alone without some form of explanation, else it will be no different than Sally throwing herself at Toby. The key is to remain subtle enough to give the reader a chance to ponder it themselves and need to know more, without being so abstract that it makes no sense and becomes too jolting.
As with so much in writing, it’s all quite a balancing act. The scene where the character steps away from their established persona needs to fit into the rest of the plot, to act to enhance the plot and give it some progression. It also can’t stand alone and be ignored; it should be referenced and built upon after the moment so that it serves some purpose. What good is Zach’s mention of his past and saving the puppy if it’s entirely forgotten in the next chapter?
There are times where it can be done without surrounding information and hinting, though. Breaking from the character’s persona and known paradigm can be used to jolt the reader if it’s a sign of a mental shift or some paranormal problem. The sudden shift can be a sign that a character isn’t dealing with a previous trauma as well as it had originally appeared; perhaps there’s more going on behind the scenes that the reader doesn’t know about yet. A previously aloof, quiet character could become snappy and aggressive where they’re suffering under the stress. An outgoing party girl could become withdrawn and obsessive. These are signs of mental problems that can build and be expanded upon. They could also be a sign of something paranormal occurring, if written into speculative fiction.
Of course, it could be a happy change as well. A previously quiet and nervous girl could gain confidence and a big smile on her face. Perhaps a cold man becomes flirtatious and bouncy. They could be signs that something big and joyous has gone on behind the scenes. Maybe rather than a traumatic event, the character has had some big positive news that they can’t share just yet.
In either case, it’s much easier to have the sudden shift in a secondary character’s personality rather than the protagonist. It’s difficult to hide something big like that from the reader when they are effectively living in the protagonist’s shoes. It can be frustrating to the reader to have something that carries such weight knowingly hidden from them. They should know everything the protagonist does. It has potential to be pulled off with something paranormal such as a possession or a demonic bacterium or some such; something that the protagonist isn’t consciously aware of. That way, you’re not hiding something from the readers, as the protagonist themselves isn’t aware of the problem or the changes. It could be used to hint at something being wrong to the reader, but it still needs to be handled delicately so as not to confuse your readers too much.
In summary, don’t break away from the character’s established persona unless there is a good reason for it and it can be built upon. There needs to be something within the character’s history to explain and act as a foundation for the character’s shift and reaction, unless it’s a paranormal thing. There should also be hints surrounding the shift and actions progressing from it to give it purpose within the plot. That is to say, don’t allow it to stand alone, use it to enhance the character development and move the plot forwards. If you really want to have it be a big shift with no surrounding information, then it’s much easier to do that with a secondary character where some information can be hidden from the reader.