On-the-fly test plans

I'm trying to encourage testing as an inquisitive, intellectually-engaged process. While test cases, test-case management software, and excel spreadsheets can be useful, they can be counter-productive if they take the place of good detective work. In my experience, they serve as a reminder of what to test, or more accurately, the things you jotted down last time you considered what you should test and jotted things down.

I wonder if, in addition to or replacing entirely a bank of static test cases, it would be better to create on-the-fly test plans/cases. This would consist of opening up a blank spreadsheet, entering all of your assumptions about what should be tested, making a test matrix, deciding what portion of those tests should be tested, and reviewing your test plan briefly with a developer who cares.

This should not be interpreted as a linear or step-by-step strategy. These are just some of the elements thrown together roughly in the order of the first occurence of each item. Ideally, a developer or customer will be available throughout the process for clarification. You'll be seen as bugging and bothering people until you uncover some miscommunicated assumptions whose discovery saves everyone a lot of trouble. You'll build a reputation for heading off trouble before it troubles others, and developers will hopefully be more willing to be interrupted to answer questions.