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.

SISTER EVANGELINA (Pam Ferris)

SISTER EVANGELINA is the only one of the sisters who comes from the same tough, uncompromising background as the community they serve. Physically vigorous, she has a robust sense of humor. Her energy and drive make her extraordinarily successful at her job. However, she does not suffer fools gladly and her blunt speech often offends. She has a particularly contentious relationship with the aristocratic SISTER MONICA JOAN, who often provokes her to breaking point.

Pam on Sister Evangelina: “Evangelina has an interesting history which doesn’t appear in the actual scripts. She was parachuted onto the front line of the First World War because they needed nurses at the front and the only way they could get them there was to parachute them. They had to do it without training and the survival rate was fifty percent at that point, so that tells you something about the courage of someone who can be dropped at the front line of a massive war with a 50/50 chance of surviving.”

“The other story about her that really affected me was that there was a point where she had to get to a patient who was on the other side of the Thames —this was in her later years when she was about 60 —and she found that bridge was open to let a boat through and she needed to get across quickly. So, she whistled up the barge drivers and they lined up and she leapt from barge to barge to get across, and you think, what kind of a woman is this? Absolutely fearless, courageous, single minded. I just think she’s wonderful.”

SISTER MONICA JOAN (Judy Parfitt)

Born into a leading titled family, SISTER MONICA JOAN was one of the first women in Britain to qualify as a midwife. In the closing years of the 19th century, this was seen as a radical act. Her additional decision to become a nun scandalized her relatives, who never came to terms with her life choices. She dedicated her entire adult life to providing midwifery services to the poor of the East End. Now in her 90's, SISTER MONICA JOAN has retired from practice, but lives full-time at Nonnatus House and cared for by her fellow sisters. She has an eccentric, mercurial personality, and is obsessed with cake, astrology and knitting.

She has a very different nature than SISTER EVANGELINA and misses no opportunity to bait her. It is never entirely clear how much of SISTER MONICA JOAN's eccentricity is due to the frailty of age or, as JENNY suspects, sheer willful naughtiness.

Judy on Sister Monica Joan: “She comes from a good family and she’s an innocent. She’s very eccentric but she’s also getting dementia so she’s in and out of reality, which makes it really interesting to play because it’s a very difficult line to get through.”