@Letty,
He also began to make more black-and-white films and take on roles actually portraying characters sometimes much older than he was -- the psychiatrist in "Suddenly, Last Summer," and "Freud." It's a pity he ended up in a New York brownstone, totally afflicted with alcoholism.
"The Heiress" is my favorite Monty film even though he plays a cad. Of course, then he went on to play a murderer in "A Place in the Sun," the most gut-wrenching of all of his films, portraying the sad destiny of love.