Be professional. Ideally, before a session you will have done some homework, by reading the synopsis, thinking about the gaps, and if necessary doing some research on the company, the product or the clinical area.
Don't try to be a clinical expert - you're there to help the client think about the evidence they need, not write a trial protocol for them. At worst, you could be wrong and lead them to wasting a great deal of time and money.
Don't skip uncomfortable truths. You will not be doing them any favours if you simply tell your client what they want to hear. Many developers leave HTA considerations to last (if they think about them at all) so you may need to tell them things they'd prefer not to hear.
Finally, don't forget - it's a gap analysis. It's neither possible nor appropriate to try to solve all their problems. If you can help your client see the medtech landscape from the HTA perspective, and to really understand what evidence payers need when making decisions, then the META facilitation session and report will have served their purpose.