Rule reminder
Near-synonymous periphrases differ in nuance: 'mettersi a' implies a sudden or resolute start, 'cominciare a' is neutral, 'stare + gerundio' indicates an action in progress at a specific moment, 'andare + gerundio' suggests a gradual or ongoing process.
The old professor had the habit of starting each lecture with a story. Formal register. Which is most natural?