I would assume gate agents check with flight attendants before starting boarding, but I guess that’s not always the case?

Soemtimes I was the first one to board, only to find the flight attendants were still settling in and asking me to wait in the jetbridge momentarily.

No big deal until an unfriendly encounter tonight. The purser seemed late (other FAs were ready while she was still taking off her coat). She was apparently annoyed to see us passengers. Maybe she had a bad day or just hated her job. We had to wait over an minute standing outside watching her taking stuff out, putting stuff in and storing her bag, all in super slow motion... like an SNL sketch.