For example, last week I read Ellis Peters' One Corpse Too Many, one of the Brother Cadfael mysteries. I've seen a bunch of the Cadfael episodes on PBS, so the image of Derek Jacobi as Cadfael was already firmly lodged in my mind, and that image does seem to fit with what's on the page, at least until I try to imagine Jacobi/Cadfael as an ex-soldier/ex-sea captain. The book, itself, was fine, except that if you already know that Hugh Beringar can't possibly be the murderer, you're really only left with one suspect. Also, there's a very nice map of the Shrewsbury area in the front of the book, but I'm not sure Peters ever actually looked at it. I did just obtain the rest of the series, so I'll no doubt read more of them.
There are times when it doesn't work at all. I got into Nero Wolfe through the TV series that ran in the early eighties, and when I started reading the books, the image of William Conrad as Wolfe meshed perfectly with what I was seeing on the page. However, Lee Horsley as Archie Goodwin? That I was never able to reconcile. (Tim Hutton, however, was dead on in the recent A&E series.)
I'm having a similar problem now with Raymond Chandler's The Long Goodbye. I'm trying to adapt the images I have of either Bogart or Mitchum to Philip Marlowe as he's written, and I just can't do it. Marlowe is an educated man, and neither actor comes off on screen as educated enough for me. I'll admit,though, that it's been a long time since I've seen the movies, so I don't really remember their actual performances as Marlowe all that well. I'm going mostly by their other roles. Mitchum's Marlowe is a little fresher in my mind than Bogart's, but that's not saying much, considering I completely forgot which Marlowe film he was in.
The book was confusing me quite a bit early on, because it didn't seem to match what little I remembered of the movie at all. I remembered Robert Mitchum's narration about Joe DiMaggio's streak, which occurred in 1941, running through the film. The book I was reading was clearly set in the early fifties. Charlotte Rampling's character didn't seem to be there, either. So I went over to IMDb and looked it up. Okay, the movie I was remembering was Farewell my Lovely. D'oh! The film version of The Long Goodbye has Elliott Gould as Marlowe.
Blink... Blink... Trapper John... Papa Geller... Nope. Still can't imagine it. I'll have to watch this movie someday.
I do like the book a lot so far, even though I am having a hard time following Marlowe's reasons for befriending Terry Lennox. Oh, yeah, in the weird coincidence of the day, the part of Terry Lennox was played in the film by Jim Bouton, of all people, whose Ball Four is my currently sitting on my nightstand. He does look a bit like the description of Lennox, but Bouton sure doesn't speak with Lennox's faux English accent.