I've done this for a long time, too. I remember reading Night of the Twister in the late 1980s. I was eight or nine. I pictured the kids in the book as Kevin, Paul, and Winnie from The Wonder Years. That didn't even fit the descriptions of the characters, but somehow it rang true enough to me that I merrily disregarded the author's own vision.
I'd like to think I've at least gotten more accurate. The latest celeb who has been acting out novels for me is Owen Wilson playing John Sandford's Virgil Flowers. Here's Sandford describing Virgil in Rough Country, the first Virgil Flowers novel I read*:
Virgil was lanky and blond, a surfer-looking dude with hair too long for a cop, and a predilection for T-shirts sold by indie rock bands....That's all it took. For some reason that sounded like Owen Wilson to me, and it's stuck throughout the three Virgil Flowers novels I've now read.
One more curious thing: this never, ever happens while I'm writing. To me, my characters either look like the people I'm basing them on, or like completely new people as I've written them. But it does make me wonder, on the off chance that others share this quirk, what celebrities a reader might cast in my work.
(*The first I read but the third of the series. To be fair, I may have started out of order but I did circle back to the beginning.)