There is a reason why first impressions are important. As soon as you start your testing, there’s no going back! You will make immediate choices and instant appraisal about the software which are almost impossible to change.
Once you jump into the details of a story, you will have already lost one of the most valuable testing assets you possess; an open mind! It will be almost impossible to view that story with the same sense of impartiality and optimism you had prior to beginning. This critical time period is when testers do their best work. When testers are freed from the narrowed thinking caused by focusing too heavily on the details in the early stages.
Speaking to a developer or product owner before you begin will help clarify expectations and provide guidance. However, other times, these types of conversations can cloud thinking and inhibit your intuition. Once a dev or PO has rationalized the intent behind the new feature, your ideas have been unintentionally sanitized. You can no longer view this feature like a customer would when they first interact with it. This “rationalization” has compromised your testing. You are now longer observing with an open mind but with preconceptions and expectations, albeit well intended.
Before you begin your test session, figure out how best to structure it. I would advocate reading the title (or maybe a brief summary) of the feature and time box yourself to 5/10 minutes. These few minutes will become your most valuable testing time. Once your time box elapses, you can then go through the detail of the feature and write out and execute all the required checks.
A tester’s job is to observe and inform, not perceive and judge. Don’t let your presumptions get in the way of how you see things. Your first impression is a precious commodity and needs to be handled accordingly; with extreme care.