Writing unit tests on Legacy Code is an adventure. Today I spent several hours doing that with two developers, Mark and Yelena.
The system has a flow you are used to and when you write code you fit it into this flow. Then you test the entire system.
With unit tests, it works differently. You are required to start the flow from the middle and stop it when your deed was done.
This requires a different level of system understanding. When we started off in the morning we looked at the code and thought “how are we going to harness this?”