In the part of the movie When Lestat is yelling at Louis about not knowing anything about his maker (Their in the dining room) Lestat starts yelling while standing up and a long string of spit comes flying out of his mouth.
I don't consider the drool a mistake. I consider it Tom Cruise completely immersed in the role. It almost seems like you have to say "Tom, come back, you're not Lestat.". Ya Know? I've always noticed the drool and I think its cool!
He's a vampire, according to the book I don't even think Lestat would have saliva(if you read by Anne Rice's rules). This in it's own way IS a slip up, and it's own way ISN'T.
Tommy is obviously having LOTS of fun as Lestat and is VERY into his role -- it's all apart of the act.
But Lestat probably doesn't have saliva, at least according to (affectionately dubbed) Annie's Vampire Laws.