Rule reminder
Register is determined by audience, medium, and purpose. A tweet can use any idiom; a legal contract cannot. A conversation with friends welcomes color and humor; an academic paper requires precision and neutrality. For each context, ask: who is reading/hearing this, and what level of formality do they expect?
An HR manager is writing an annual performance review for an employee. She wants to say the employee works very hard. Which version is appropriate for the formal document?