How do I find values that share the same keys in a dictionary?

I want input to be actorsInCommon(movies, "Moneyball", "Oceans Eleven")

I want output to be ["Brad Pitt", "Joe Smith"]

movies = {"Moneyball": ["Brad Pitt", "Jonah Hill", "Joe Smith"], 
          "Oceans Eleven": ["Brad Pitt", "Joe Smith", "George Clooney"]}

def find(key, dictionary):
    for (k, value) in dictionary:
        if key == k:
            return value
    return None

def actorsInCommon(dictionary, movie1, movie2):
    movie1Cast = find(movie1, dictionary)
    movie2Cast = find(movie2, dictionary)
    return list(set(movie1Cast).intersection(movie2Cast))