Great question and I agree with the responses from Erja and Tanya.
One thing that is important to discern is where you are in the process. My assumption is that you have an agreement in place and are beginning the post-merger integration.
If that is the case, then I would want to look back at the due diligence conducted to better understand the deal thesis and the why behind this. To Erja's point, I'd want to see what language has been used to communicate this and what channels are typically used for employees. I'd look to build a governance structure that includes an integration team. To Tanya's point, a Culture Assessment will be key in determining how operationally ,financially, and culturally alike the organizations are and where those leverage points are and where there are potential risks and challenges.
This will help inform change impacts and can be done at the organizational level and at the divisional/functional level. You'll want to consider the org design - where there may be overlap, synergies, and opportunities to align the business model.
For a lot of your questions - it is an it depends on the role, scope, scale, and speed. The core integration team will be heavy with this being pretty full-time and you'll want designated business leads to support and get more engaged throughout the integration.
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Evan Piekara
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