I agree with the other comments that it’s probably about memory more than anything, especially in faster paced communication like an in person conversation.
In like, a blog post intended for lots of people to read, it’s contextually dependent. Probably? Like, if they’re reviewing the movie, they’re talking about how the actor filled the role. In a plot description, it’s more natural to say the character name. If the actor is named then, it could be intended to imply the actor never disappeared into the role. “Then Chris Hemsworth snuck into Whosville dressed as santa and stole all the presents with his dog Max” implies that the person talking never entirely saw Chris as the Grinch.
But, yeah, in immediate one on one talk, I assume it’s the name that comes to mind first. The famous person is memorable. Though, if you got a very strange mental image, you can also consider the possibility that the speaker was making a joke, like I was with “how chris stole christmas” up there. The quality of the joke is separate from whether it was intended as one.
Text. I think the answers on this kind of social media are going to skew that way.