To be fair, sometimes my mind insists on playing by 'real world' rules when asking 'who would play so&so if this was a movie?' By which I mean, I try to imagine if this book were filmed now, who would be in it?
Does that apply if you're reading, say, a TOS novel set during the original series? If you read a Shatnerverse novel with the older versions of the characters, do you use present-day Shatner and Nimoy but recast McCoy?
I do often try to imagine current actors for new book-only roles, but I don't like the idea of mentally recasting characters we've seen onscreen.
I mean, I just read To Catch a Thief, a 1940s set detective novel by Robert Kaminsky. Cary Grant plays a large role in it - as in, playing himself.
Odd, since Cary Grant starred in Hitchcock's 1955 film To Catch a Thief
, based on a book by David Dodge. Oh, wait, you said the book is set
in the '40s, not written then. I guess the author is making an homage to the film in some way.