I used to work for a high end hotel and Jennifer Aniston stayed with us a few times. Once I was sent to check the internet in her room. She was really nice, asked a couple of questions, and then even said, “Thanks, honey,” as I was leaving the room, exactly like Rachel on Friends: In a very kind, sweet, but distant way.

She stayed with us a few more times and was always the same: Relaxed, nice to staff and other guests.

She is a real sweetheart.

Edit: To answer the question: Interacting with her is like doing a cameo on a Friends episode. It's hard to tell if the role was written specifically for her or if she created the character herself.

One couldn't help but wonder when Ross, Chandler, Phoebe, Joey, and/or Monica were going to show up.