Who are the top stakeholders that would ideally inform and be part of the messaging development process?
There are a lot of stakeholders. Are you ready for it?
Product Managers: I like to start with a PRD (product requirements document) to really understand what's in the product, the jobs to be done, how a user uses the product, and what the benefits are. This is the baseline for the first messaging iteration, and then I review and tweak with PM from there.
Sales: Once I have something to get reactions from, I bring it to sales teams. They're in front of customers every day and will be able to gut-check the language and how much it may resonate with customers.
Customers: Customers are your best message testers. It's always a good idea to have some go-to customers you can reach out to and get feedback from. If you don't have that, try focus groups or surveys (but these can be costly!)
GM and PMM/Marketing Leadership: Once I've done a round or two of feedback and revisions, I bring it to my exec stakeholders for feedback. Ideally, I've done a first pass with leadership so we're all on the same page, but at this stage I'm seeking final review and approval.