TV/films give us something to relate to , although not always accurately. How many of us who have read The Dark Tower series could visualise the gunslinger as anyone other than Clint Eastwood?
There also the novelists who helpfully show their characters on the front cover of their novels; Pratchett is good for this. One can only assume that he has approved the cover and has given his own input, so the characters look as he intended them to.
Having watched the Sharpe films before reading the novels, I simply cannot see anyone other than Sean Bean in the title role; same with his companions. This is slightly at odds with how the character looks on the covers of Cornwell's books which I would imagine would have been his vision of the fictional Sharpe, where the character is more thin, gaunt looking (which I guess most of Wellington's under-provisioned army would have been).
On the other hand having read the Hornblower series of novels before watching the tv series, I cannot relate Ioan to the character of Horatio. Having said that, the tv series bears little resemblance to the novels (or indeed the way the Navy operated). The ITV version is more like a naval version of the Sharpe series, which I guess it was trying to emulate the success of; but this was at the cost of either keeping to the novels or to realism.
So I guess that if your novel is televised then it depends whether you read the book before you watch the tv/film. Or at least that's the way it is for me. For instance, if I read The Game of Thrones novels (which I'm sure I'll get around to eventually) I doubt I will be able to visualise any of the characters other than by the actors who portrayed them on tv.