As readers, are we so saturated with the visuals of tv/cinema that we are no longer able to visualise characters without them being 'cast'?*

Last night I finished reading The Da Vinci Code, a novel which informs the reader very early on that the hero "looked a bit like Harrison Ford". A tweed-wearing Harvard professor who gets involved in a quest for the Holy Grail? To quote from a favourite movie, "It's a bit obvious, Robert." (I know, I know. I am alone in loving A Life Less Ordinary)

Then I spent a little time reading through something for a friend. My feedback included comments about casting the characters. At which point I realised I have yet to cast my beloved John Cusack in Warring States. For History 101, he was Sabbath's agent and his mental casting caused Joan to be cast as Elena. Because all the best John movies have Joan as well (Grosse Point Blank, High Fidelity). This time round, I've mentally cast Joan but not John and last night I drifted off to sleep running through my cast of characters to find one who would be plausible as John Cusack. None of them are. But, and this is what I'm wondering, does mentally casting the characters actually damage the book? Even if you do make some effort and actually describe them (unlike the "like Harrison Ford" line in The Da Vinci Code)? My main characters aren't cast this time and, since I'm writing with nods to Victoriana, I actually pause to describe them. Some of the supporting characters are real people, or have been mentally cast, and when I look over their introductions, I skimp on the description.

Now I'd think this is just my own writerly stumbling block, possibly due to me having started writing in fanfic (a genre in which the characters are already cast, by definition). Yet on places such as Outpost Gallifrey, the "who would you cast as...?" question comes up from readers frequently. I did mentally cast Anji and Fitz for History 101 (the woman from the AA advert and Enzo Cilenti, respectively) but now I wonder if that was my mind playing to the idea that novels should, in some way, be transformed from prose on the page into a little mental movie in the heads. Is it just a side-effect of writing within the action-adventure genre? I'm trying quite hard to make Warring States a novel rather than a novelisation of a film that's in my head, but it's very easy to slip**. As the time I spent last night trying to find a role for John Cusack illustrates.

*I appear to have started out like Carrie Bradshaw. Only not about about sex. Or cities.
** this is not me slyly warning the money editor that my schedule is so tight I'm in trouble...I did that the other night.