As noted earlier in the section on Thinking, essay writing does not proceed in linear fashion, thinking → writing. Instead, thinking and writing are intertwined and mutually dependent activities. As you write, you will inevitably modify and develop your ideas. You may come up with additional evidence or arguments, or problems you hadn't anticipated. As you pore over your textual material, you may even start having new ideas in a quite different direction.
This happens to all writers. It's actually a good thing; it's one of the ways writing helps you think.
You do of course need to end up with a focused essay, but that may not mean slavishly following your original plan of attack. Don't force your material to fit your argument. Instead, expect to reshape your argument to fit the material. It's remarkable how often students argue simplistic points in an essay, as if they feel trapped by what they decided to say in the first paragraph. You're never trapped: as you write, you think, and if you need to go back and rethink your thesis, do it rather than make a poor argument.
This happens to all writers. It's actually a good thing; it's one of the ways writing helps you think.
You do of course need to end up with a focused essay, but that may not mean slavishly following your original plan of attack. Don't force your material to fit your argument. Instead, expect to reshape your argument to fit the material. It's remarkable how often students argue simplistic points in an essay, as if they feel trapped by what they decided to say in the first paragraph. You're never trapped: as you write, you think, and if you need to go back and rethink your thesis, do it rather than make a poor argument.