Horror films are just what the doctor ordered. So contends British actor Christopher Lee, whose idol is Boris Karloff, and who has played Dracula, Frankenstein's Monster, Fu Manchu, and various other evil and sinister gentlemen. Lee is playing Dracula for the second time in "Dracula, Prince of Darkness," which opened in 1966. Hammer production for 20th Century-Fox release in DeLuxe Color. "According to doctors I've talked with," Lee says, "horror films are a form of nightmare. They reactivate people's fears, give them direction, and then dispel them. This provides people with a valuable emotional outlet. "Also," says Lee, "horror films are pure fantasy. They take people away from themselves and their problems. After all, not even the most ultra-sensitive person can identify with Dracula. "And talking about Dracula," he continues, "I'm getting to like the old boy. He had a flair for the dramatic, dignity, and a strong sense of survival, too, like any sensible man."