Possible Clues:

• Possibly play song with bounce relevantly somehow passing on G.B.S.
• It's always impossible to predict what you might see in the theatre
• Uncertain consequences of joining MI5?
• It's unpredictable, but don't ever say so
• Characterised as pleasant play by its author (but it's impossible to be sure)
• Exclamation on the unpredictability of life