SPOILER!!!! Need help understanding the end of this

I liked the plot twists in this movie, but the last twist confused me. The producer was murdered...we think we've finally learned the killer (the playwright)...but no, it was the actress. Something about love letters and blackmail. Did the actress shoot the producer because he was blackmailing her? Why was she at the producer's home that night? Why was she in the playwright's room at the end?