Well, I have never come across an interview with AR or director Neil Jordan about why Antonio was cast for Armand. I should have an FAQ and when I do, this will be right up at the top of that list! 

I should probably watch IWTV with the director commentary again; he might have mentioned smtg about it and I’ve just forgotten, so I will do that.

ANYWAY, there are a bunch of theories as to why Armand is portrayed the way he is in the movie and my fave explanation, currently, is that movie!Armand is actually a blend of Armand and Santino. 

Why do that?:

• If they had cast a cherubic teenage redhead (or older creepy cherub dude trying to look 17 *cough cough* movie!QotD *cough*) it might have made Louis look like a pedophile, dumping his 11 yr old “beloved” (as he calls her) for someone only 6 yrs older (in appearance, anyway).
• For people who hadn’t read the book, someone cuter than Louis might have taken the sympathy away from him (who was supposed to remain the focus, it’s HIS story, after all). That would have been compromised by omg cute little redhead bby we want to snuggles you! 
• Armand was supposed to replace Lestat in the sense of being a better mentor/teacher/father figure to Louis, and so that someone had to be more paternal-looking. Someone who would look older.
• The actor had to be a bigger star than Brad Pitt (or at least older than BP)
• And he had to have some European or other ethnicity, because these Parisian vampires were supposed to be darker, scarier, more like Real Vampires of the Old Ways than the softer Americanized Vampires. Their vampire makeup (aside from their TdV costuming) shows them to almost be a different, more vicious species from Louis (who has to still maintain his “most human”-looking vampire status).

In all this, Santino really fits the bill better, and my guess is that AR suggested it, she was the screenwriter, after all. 

For more on this, check out my #Defending Antonio tag.

Also go check out this memeything by @luthi69​ bc it is relevant.