Perhaps when you annotate your thesis to get ready for the viva you won’t strictly write “between the lines” – but you will need to consider what things mean and draw something out.
Maybe you’ll spot a keyword or two that could be highlighted. Or a sentence that stands out or needs a note in the margin for clarification.
As you prepare you’ll probably want to write a summary or two as well. You’ll consider a section and “read between the lines,” capturing the big ideas or points in a few sentences.
There are a lot of words in your thesis. You don’t need to remember them all to succeed at your viva, but carefully reflecting on what’s there, looking between the lines, is a valuable investment of time as you get ready.