I thought that it was impossible to keep your witnesses a secret from the other lawyers during the trial. Don't they have to be open and up front about it and provide a list of all the people they are intending to call to witness?
I know very little about law, so I think I got that out of a movie or something. For a long time I watched a lot of legal shows, although they seem to have become less popular now. They were always calling secret star witnesses to the trial.
I particularly liked Boston Legal and was sad when it ended. It was all the best parts of Ally McBeal and The Practice mixed together with its own kind of intelligence.