A good contractor can be expected to know what must be done before the project begins. However, there are many kinds of unanticipated problems and challenges that can be encountered only after the demolition starts.  For instance, while it makes sense to expect a contractor to know that removing a load-bearing wall will require temporary support, it might not make sense to expect the contractor to know that removing drywall might reveal sub-standard framing that will need to be corrected.