Fan Casting The Renfield in Sort by Tropes

Vampires are usually bad for human health, especially the evil ones. But that doesn't stop certain people from going into a vampire's servitude. Part of the reason is, when you can't go out in sunlight, there's a lot of things you can't do for yourself. Part of the reason is that sometimes you just need minions. Enter The Renfield. Reasons for this can vary; either the slave has willfully gone into servitude, the would-be slave got addicted to the vampire's blood, the vampire used some sort of mind-control power, occasionally they'll just be hired, and/or the vampire simply tempted the would-be servant with the possibility of becoming a vampire as well.