Wow, i don't know how i'd react if such a mad man was standing in front of me. That guy is a hero indeed.
Hopefully you don't freeze in shock.
If there is enough space, you want to run. Use everything there is as a barrier between the attacker and you. Cars, trees, fences, tables, stools. If you are in a closed space with nowhere to run, try to get a (improvised) weapon, best something big which creates distancs like a bar chair, trash can.
If there is nothing available and you have enough time you want to use your jacket to help your defense. Hold it like a Spanish torrero in front of you, that creates a psychological wall between you and the attacker. He can't see your whole body anymore which makes it harder for him to aim for his attack. Also if he thrusts you may try to trap the knife hand with your jacket.
If there is no improvised weapon available and no time to make your jacket ready, you might try the Jon Fitch style approach as last resort:
Wait as long as he is not engaging and in your range. As long he is not in range he cannot hurt you, but as soon as he gets into range go for his knife arm with BOTH of your hands. Some people say that you shouldn't, because that leaves your opponent one free arm to act, but first the knife is your primarily concern and second if he is not a trained fighter he will most likely be absolutely focussed on his knife and not use his other limbs as weapons, because subconsciously there is a huge disbalance in the effectiveness of dealing damage between a knife (weapon) and his bare fist. You can try that out in training. Even with a rubber knife in training people will mostly try to get the knife arm free instead of attacking with their free limbs. That happens even in the Fitch clip above.
While controlling his knife arm you want to stomp kick at his legs trying to break his knee or what ever is best available. You want to immobilise him as much as possible due to damage and look all the time for an opportunity to get away.