I know in my sitch BM and I get along well because we both always have our guard up and on our best behavior. We are civil and can talk about SD, or easy stuff like books or movies.
When SD was getting a psychiatric eval and admission to the hospital, we (DH,BM and I) were forced together in a room for about 5 hours. DH and BM had taken SD together but called me to come up. I even stopped and got us all dinner.
During those 5 hours BM was totally "real". No fakeness no walls. She talked to DH and I like we were friends. We talked about lots things and got to "know" each other. We even swapped birth story's and it was nice to hear about SDs birth from her POV. She was also pretty funny.
We all went back to being on guard the moment we left the hospital, but is was nice seeing a glimps of the "real her".