Helen Mirren had director approval. She had the the right to look at directors and meet them. She met Tom Hooper and approved him, telling him it had to be his vision and she would protect it.
Towards the beginning Tennison says she is 54 years old, however in Prime Suspect 5 she says she is 50. As Prime Suspect 6 is set 7 years later she would be 57 not 54.