On Becoming a Guinea Fowl, Zambian writer and director Rungano Nyoni’s second feature film, is a highly accomplished ensemble drama about family secrets. Secrets don’t just reveal themselves right away, and the film’s style is accordingly a matter of glimpses and intuitions. Shula, a young woman played by Susan Chardy, is driving home from a costume party, still wearing her elaborate disguise, when she finds a dead body in the road. When she gets out of the car to look at it, we briefly see a young girl in her place wearing the same costume. Back in the car, she…