During training, it is very important to practice realistic attacks. If you punch someone and miss or have it blocked, you pull your arm back. At the beginning of our training, the arm is often left out there while you learn your defense. One of the criticisms from the reality based martial arts camp is that traditional martial arts don't always move past this phase.
Once we understand the mechanics of the defense, we need to make sure we account for what the attacker would most likely do. We need to respond instantaneously or use a trapping technique to defend realistically.
What is equally important is what to do when you receive the technique or the defense (being an Uke.) One of the first lessons we learn in Jiu Jitsu is that the body follows pain. To be a good Uke, when a technique is applied to you, be it a strike or joint lock, you need to respond as realistically as possible. If we're punched in the stomach, we need to double over somewhat. If we get poked in the eyes, we need to pull back.
Jiu Jitsu is about redirecting energy and using your opponents force against them. Distraction techniques are used to divert your opponent's attention from what you are about to do. As an Uke, if you don't respond to the distraction, you are doing your partner a disservice. He/she will not respond realistically to you and follow up techniques will be limited. By responding to the softening up or distraction techniques, we force our partner to adapt to where your body would most likely go as a result of their actions.
I'm reminded of skeptics that ask you to do some Jiu Jitsu (or any martial arts) technique on them. You attempt to show them a simple technique and they tense up using every ounce of strength to try to resist it. They look at you with that look that says "See, it doesn't work on me".
Sometimes you try to tell them you'd use a distraction technique or to stop resisting or they'll get hurt. They're usually not listening, believing they've proved it doesn't work.
On one such occasion, I was trying to share a technique with some of my co-workers. One cocky lad was set to prove me wrong. I was demonstrating a reasonably simple technique from a handshake. He was set to crush my hand and I could tell he was convinced nothing would work on him. I looked him in the eye and then kicked him in the shin (not very hard). His attention went to his shin and his face had a look or surprise on it. I easily finished the handshake technique on him, dropping him to his knees. He wasn't hurt, of course, but he was impressed. He now takes martial arts.
In training, we need to respond as realistically as we can, both as the attacker and as the defender.