The British chat show host is friends with the 'Skyfall' hitmaker after they met at the Brit Awards in 2008, and he insisted she is still completely grounded despite her huge global success.
Speaking to The Sun on Sunday newspaper, he said: ''She is what she is.
''She's the most down-to-earth person you'll ever meet and hasn't changed at all, despite all these Grammys and Oscars coming out of her beehive.''
Alan, 37, has become very close to Adele, to the point where he admitted he feels pressured with the amount of secrets he knows about her private life.
He explained: ''I knew about her pregnancy and her son's name before it came out, but I'd rather I didn't.
''I have to plead with her not to tell me any personal secrets because there's always the risk I'll have a few too many wines, get on Twitter and put the world to rights.
He also revealed his first meeting with the star wasn't ideal after he fell into a lift in a moment reminiscent of British sitcom 'Only Fools And Horses'.
He added: ''I was very drunk and saw her sitting on a sofa, so I called out from where I was standing, precariously leaning against what I thought was a wall.
''But it was a lift - and just as she looked up I did a 'Del Boy' and fell straight through the doors, smashing my head against the floor. I was straight home with a concussion.''
American Thighs was released on this day in 1994.