I'm not sure I understand what you mean with "hostile witness". I am pretty sure that if you ever find yourself the main focus of the legal system, you will face questions that are loaded, twisted and made to appear YOU look guilty. And not only from an attorney:

The police will also ask you loaded and leading questions, see TheSock's examples or the video.

If you are in deed innocent you need to realize that the police do not think so - or they would let you go. If the police think you did something it is their job to gather evidence to prove that you did it. You may think they also should gather evidence that you're innocent, but that's not how it works. They work as hard as they can to blame whatever wrong that's been done on YOU. Your mouth blabbing away only makes that job easier.

I'm all for being an honest and helpful citizen and helping the police. But you need to realize that if you find yourself in incriminating circumstances the police are not your friend.

That being said, denying the on-scene officer vital information seems rather unproductive, too. Three of them, went that way, one armed with whatever

EDIT: OldSoldier brings up an important point - your legal status and your obligations as a witness versus your legal status when under arrest. Yet another reason to bring in legal councel. It is not about obstructing police work - it is about not worsening your situation by talking yourself into traps set up by professional interrogators.