How does characterization affect the plot?
Because ultimately the characters form the plot.
Imagine there is a person A.
A- is really intelligent,polite and calm but very scared of public speaking. You haven't shown that he has or will have any other traits.
Now keeping in mind of there characteristics of the of character only will be make a plot right. The plot should be able to maintain the flow of the character,if its too absurd and the true characteristic are not seen of the character then the reader will find the plot too weak and become uninterested.
You can surprise the reader without being absurd too. Like you can make A in your plot overcome his fright of public speaking, but in such a way that everyone should be like-yes come on A ! you can do it!
But you can't just show that one day he is suddenly speaking on stage very well (unless it is time lapse )
People like to connect to characters, keep them real, and its better to build the characters on values and limitations ,something that they would or would not do.
Don't let your your minds limitations become your characters, people can see right through it, they say the character is not at all like that! the writer didn't keep in mind this and that.
Which is true.! It happens a lot of times
build plots that show all sides and improves the character.
the characters are your story!