I am finding difficult to understand the question. (Sorry English is not my first language.)

That is meant by " average imdb.rating for each cast member."

imdb.rating is for the movie and it has many cast members. Do we have divide to by the number of members and add to people? (e.g. is a movie has rating of 3 and it case 3 names in cast, each one will get one vote) Is sum of (rating multiplied by votes) and divided by the number of votes (e.g. movie 1 have 100 votes and rate is 4 and movie 2 has 300 votes and rate is 5 average rate is (400+1500)/400 = 4.75)

or just add all the rating where each actor is mentioned and find the average?

Is it possible to give more details.