What makes an actor a big box office draw? Is it fame alone or is talent required? Usually that question is confounded: it’s hard to rule out that an actor became famous because he is talented.
The actors playing Harry, Ron, and Hermione in Harry Potter and The Deathly Hallows are most certainly famous, but almost certainly not because they are talented. They were cast in that movie nearly 10 years ago when their average age was 11. No doubt talent played a role in that selection but acting talent at the age of 11 is no predictor of talent at age 20. The fact that they are in Deathly Hallows is statistically independent of how talented they are.
That is, from the point of view of today it is as if they were randomly selected to be famous film stars out of the vast pool of actors who have been training just as hard as they from ages 11 to 20. So they are our natural experiment. If they go on to be successful film stars after the Harry Potter franchise comes to an end then this is statistical evidence that fame itself makes a Hollywood star.
Here’s Daniel Radcliffe on fame.