I was working with someone I hadn't met before, but I'd been gone from the dojo for a week long seminar, and we often get drop-ins from other dojos in the area, so I never assume someone is an absolute beginner...
I introduced myself and asked if he did Aikido somewhere else (since ukemi required a roll, it was my subtle way of deciding if I could throw him). "Well, not Aikido..." he answered. Since we often get folks experienced in other 'falling' MA (Judo, Jujitsu, Hapkido, etc) I persisted "OK, but you know how to roll?" as I went slowly through the somewhat complicated technique we'd been shown. "Pretty much" he answered, and just before the throw added "I read about it in a book."
I quickly grabbed him back up and said I'd just do the tech. to the point of throwing, and then he could throw me, and I'd explain a roll in real life vs book to him. He was having trouble with the technique (it turned out this was his first 30 minutes of Aikido) and our 6th dan sensei approached to help. He explained and threw me a few times. Then he turned to the student and offered "Here, feel". I managed to yell "Sensei, he doesn't know how to roll" at the same time Sensei launched him. He tumbled head-rear-side-rear-head-rear etc across the mat, got up, and wobbled back toward us. Sensei kind of peered into his face, looking concerned, and said "tap, tap" demo'ing a tap against his thigh for the new student.
'Kind of like the book?" I asked. "Um, I think I'll just throw you" he answered.