Blood for Dracula

The story is that while filming Flesh for Frankenstein Paul Morrissey and crew discovered they were quite ahead of schedule and under budget, so they decided to make a second movie. Released the following year, Blood for Dracula, which shares Frankenstein's critique of sexual promiscuity, was partially improvised and for some reason has a cameo from Roman Polanski. It's also a much more entertaining movie no matter what the Rotten Tomato ratings suggest, despite Joe Dallesandro's character being much more overbearing and hard to handle.