In the novel, character development is more in - depth. For instance, the main character Ainz has more internal monologues which show his growth and change in thinking over time. In the anime, this is sometimes not as fully explored due to time limitations.
The relationships between characters are also more elaborately described in the novel. We can see the subtle changes in how characters interact with each other and how their opinions of one another evolve. In the anime, while the relationships are shown, they lack some of the nuance and depth that the novel provides.
Some secondary characters in the novel have their own arcs of development that are either cut short or not included at all in the anime. The anime focuses more on the main characters and the overall plot progression, so it may sacrifice some of the detailed character development found in the novel.