During development they use moc's for the payloads that we're not ready to give them. When we build things FOR them all members are happy.
A good rule is to have the teams come to an agreement on the data that they need to show the user. Then have the BE team build out a rough payload.
Paired programming on features is always good.
Myself personally. I have a simple thing to keep in mind. FE keeps the customer happy, I keep FE happy, ops keeps us all happy. As long as BE remember they are there for the FE, everything flows smooth.