I think Occlumency is even more subtle than anything said here. You don't block an intruder, you mislead them. You have to have such control over your own mind that you can keep yourself from thinking about what the attacker wants you to remember.

As for faking a memory, Slughorn didn't just make a counterfeit copy of his memory and alter it; he had to train himself to remember it a different way. He forced himself to believe that he really didn't tell Riddle a thing.

Major props to Sree for bringing this topic up in the first place; the conceptions of Occlumency I've seen out there are truly irritating.

 

Faking a memory is technically possible. That's what Riddle did with Gaunt's son, he implanted a fake memory into his mind to convince him he was the murderer of Tom Riddle sr. and his wife; I guess you could do the same to yourself. You would create the fake memory and implant it in your mind before confronting the one you want to fool.

However, I'm not sure you could create a fake memory in two seconds and without using your wand, just your willpower. Occlumency is defined for me as the ability to hide some thoughts and push others to the front of your mind (if that even makes sense). If confronted to a not very skilled Legilimens (or one who isn't trying hard), you could push the right thoughts to the surface in order to make them believe whatever you want; no need to create a fake memory. But I doubt Voldemort or Dumbledore would be fooled if they were suspecting you're hiding something.

 

From what I've heard, I think the occlumency would be the ability to organize the mind. in it's basic form it's just clearing things away, sort of like stuffing all your stuff in the closet or under the bed. it won't last for long, but for that moment there is nothing to find. as one progresses they can learn to properly organize the mind not just to hide, but also to deceive. leave behind imagined situations where truths would normally be expected. a true master would not only be able to decieve his opponent or eject them, but also face them inside their mind and fight them.

Legilimency would be the ability to enter others minds. In it's basics you would be peeking, getting random bits of memory and sensation and emotion. as one progresses they learn where to find different things and how different minds would work. It's mastry would involve learning to navigate efficiently and without knowledge.