You would think that when it comes to players who are kids, say, under age 14, it should be a no-brainer for them to grasp what role play is. They pretty much own the concept of "pretend".
I know a lot of people over age 15 who feel a bit "silly" or even embarrassed to role play, feeling that it is only a kids game of "pretend.
I've had players who are "adults" even "young adults" who couldn't role-play unless they were inebriated. At which point, it was embarrassing for everyone else at the table.
I've seen other people try to explain role-play in relation to acting. Certainly, we have all seen either in person or at least on a video or something the people who REALLY get into character by dressing up and speaking in medieval Olde English as though they were about to participate in a Renaissance fair.
Role-play is not acting. Role-play is more about getting into someone else's head. Thinking about how they think, how do they approach things. It's about seeing things from the perspective of someone else. It is a tool used in psychology as well as by the modern detective in crime enforcement.
In regard to AD&D though, there is a bit of fun added to it by not only thinking as the character but speaking for them too. Interacting with other characters as though you were the character. Could this involve a wee bit of acting? I'd say that it definitely benefits from some creativity and a willingness to step outside yourself a bit.
I have a now 13 and 11 year old playing, soon to be 14 and 12 years old. Up until now, they have been playing AD&D1E for about a year now. For them, it's been more like the characters are simply avatars for their own personalities.
To watch them try to handle multiple PC's in a game is about the same as going to a play and seeing the same two actors come onstage at various times calling each other by different names. and maybe wearing different clothes.
They weren't "seeing" the PC they were playing as something separate from themselves. For people who still play "pretend" in some form or another, I would have thought this would be a natural for them, yet it wasn't.
So I decided that they needed to figure out how to role-play. I gave them a person that they knew pretty well and set up a scenario in which their sibling had done something WAY wrong and asked them what I would do (as Dad) if I busted them.
Obviously, this led to a giggle fest and a wary first attempt at describing what I would do though they seemed more afraid to really nail me as if they might get in trouble for being too good at it.
Finally, I said "show me, don't tell me." and my son, being the younger, finally "got it", stood up and said in a voice he obviously thought sounded like mine something to his sister that made everyone in the house roar with laughter because he nailed me. He got it perfect.
Then the other one got into it and within about 10 minutes, they had their head in my head. so to speak.
After that, I pointed out that that is what they are supposed to be doing when they play AD&D. Don't be themselves, be the character. Think of what the character would do and say.
It's been a few days since and they both say they are having more fun than ever. They don't always speak in fake voices, but they are stopping to consider the character instead of just doing or saying what they would do. That's the whole point of role-playing.