I'll go ahead and get this started. I have edited the original post a bit for clarity. My first thought would be to learn the app as deeply as possible prior to doing anything. This includes application structure and the process it is designed to do as well as the people that need to use it and why. Ask lots of questions about the application to be tested.
If you have a good working relationship with the developers do your best to help them make the application as test friendly as possible. This mainly involves ensuring objects in the program are identifiable by Test Complete. Ensuring each object has a unique name is a good place to start. The WPFControlAutomationId may be a good way for developers to make an application more testable. AutomationIdProperty uniquely identifies a UI Automation element from its siblings. For more information on property identifiers related to control identification, see UI Automation Properties Overview. (https://docs.microsoft.com/en-us/dotnet/framework/ui-automation/use-the-automationid-property)
Has anyone here had experience using the automation id property with a project in TestComplete?