Tips for software integration

I have been involved a couple of times in software development efforts where my small module (~20 KLOC) had to be integrated into a larger module (~1 MLOC).

Complicating factors:

• It was not possible to install the larger module on my development environment. There was no offline version.
• Interface Control Document (ICD) did not exist or was out of date the minute it was written because the larger module was also in development.