How to generate Description.tcLog before executing the next node in TestComplete?

You could also look into using the Events in TestComplete so you only need to do these calls in the events.  

https://support.smartbear.com/testcomplete/docs/testing-with/advanced/handling-events/about.html

There are many events here you can use to call functions to write data to an Excel file. There is also a OnLogCreateNode and OnLogCloseNode which I have never used but may be EXACTLY what you need.  I don't know when they fire however.