I can't really figure this out. I'm reading a book about object oriented analysis and design (and UML) where UP metholodogy is being applied. I came across a question: If you make a system sequence diagram (SSD) do you need to create an interaction diagram for every call in that SSD then?
According to me, I believe you should if it would be of value, but I can't truly relate it to Unified Process.
The UP is a good process but you should not overweigh it. Take those parts that make sense and start using them. I once started using ICONIX which is much more lightweight and helped me to get up and running much faster than RUP (Rational UP).
To answer you specific question: no, you don't need to create interactions for each tidbit. The purpose of a model it to simplify reality, not to describe it in more detail than reality itself. So you may shorten the story wherever it's obvious.