Lansbury wanted to be Milady
I watched a special on AMC going through the career of Angela Lansbury, she was being personally interviewed by the host, and when they came to The Three Musketeers she said very plainly that she was desperate to be Milady DeWinter and even went to the front office to beg for the part. They refused, telling her it was taylor made for Lana Turner, and they wanted her to be Queen Anne.
I found this very interesting because Lana Turner, in her autobiography, wrote that she initially didn't want the part. It was just another costume picture and she wasn't interested. But the more she worked on it, the more she was able to work with Vincent Price, the better she liked it. Turner now remembers playing Milady with great fondness.
I thought both ladies were very good in their parts -- and very beautiful!