The Screenwriter is obviously lying
Christopher Lee has said he refused to speak in this film because the dialogue was so bad. The screen writer claims there was never any dialogue for Christopher Lee because vampires don't need to say much. The writer is obviously the liar here.
1. The scenes where Lee is present are awkward and feels almost like a forced silent movie. It's very apparent the muteness is something the film has to struggle with. There are very apparent moments where spoken words from him would have been a relief.
2. All of Dracula's minion speak.
3. Lee has dialogue in every other Hammer Horror Dracula movie he was in. So if it was something he didn't 'need' than why is this the only Dracula film where he doesn't talk? This doesn't happen again before or after Dracula Prince of Darkness.
4. Lee made them incorporate as much of the book dialogue as possible in the Hammer Horror films. Dracula can and does speak in the book.
5. What reason does Christopher Lee have to lie about this? If anyone has a reason to lie it would be the screen writer because Lee hated his dialogue. What does Lee have to gain from lying about it? It makes no sense.