I've been having a hard time keeping some of the characters straight, so I thought some background on the characters might be helpful. The bios are from PBS.org.

Nurse Jenny Lee (Jessica Raine)

NURSE JENNY LEE arrives at Nonnatus House in 1957, at the age of 22, completely unaware of the world she is about to enter. Believing she’s accepted a job with a small private hospital, it comes as somewhat of a surprise to Jenny to find she’s moving into a convent. The deprivation of the East End is a huge culture shock for the young woman who has enjoyed a privileged upbringing in the Home Counties, and spent time in Paris before training as a nurse. However, although initially shocked by the conditions in which her patients live, JENNY comes to accept the material limitations of the world in which she works, and to love the people who live there.

Jessica on Jenny: “Jenny walks into Nonnatus House quite green to the East End way of life, but she also has quite an old head on young shoulders in many ways as she’s running away from a mysterious relationship.” “She’s young, she’s 22, and she sees all these amazing life stories happening in front of her. She’s constantly learning and re-evaluating her own way of seeing the world. I really like that, I like people who can change their opinions and she does that during the course of the series.”

Sister Julienne (Jenny Agutter)

SISTER JULIENNE is the sister-in-charge at Nonnatus House. Deeply religious, she is also profoundly practical and confers dignity on her patients through her sensitive, non-judgmental approach. She is often called upon to mediate between her fellow sisters when they quarrel, as this is an almost exclusive female world, and her tact and gentleness are essential to its happiness.

Jenny on Sister Julienne: “My mother was tended to by a nursing sister from a religious order in Westminster for many years. This nun was wonderful and radiated warmth, there was an almost childlike joy with which she took on every task. Her sympathy was not cloying but unfussy and honestly felt. I think that childlike element is there in Sister Julienne. She’s responsible and in charge, but there’s a twinkle there and a sense of fun. She finds everything she is involved with really interesting and people are rather extraordinary. Nothing is ordinary or mundane life is amazing.” “The dynamic between the older nuns is an interesting one. It might be assumed because of shared values and common beliefs that the nuns would live in perfect harmony with one another, but like anyone else they differ in their characteristics and this can cause friction.”