I've been in a similar situation once. My order of battle is: (and this isn't legal advice and I'm not a lawyer)
1) Don't lie to the cops. If you don't know, say so, but don't guess.
2) See # 1 again. You don't have to say anything, but potential suspects (i.e. you're the only one there, maybe you're the robber?) don't win any cop friends, and it is not uncommon to hear they will often help you if the initial story holds water. As in "...and you were in fear of your life and you drew to stop the threat", correct.
3) The $250 - $500 I spend on an immediate call to my attorney before providing a dissertation to the officers is significantly money better spent than waiting until after with a defense lawyer. He's likely to remind me to tell the officer(s) at a very high level what happened: BG with knife approached, I was startled and in fear, I drew a weapon, BG ran. BG was an orange haired 5-5'6" man in jeans and Tshirt. That I'm currently very shaken, and will provide more details later. That's it other than name & contact info.
4) Don't hide anything like your weapon.
But then this is all hypothetical, right?