“Leighton contends that, by filing a defamation lawsuit, the Dentists implicitly agreed to cooperate in discovery, and, therefore, their subsequent refusal to do so was an abuse of that process,” Justice Catherine R. Connors wrote on behalf of the unanimous SJC. “The Dentists’ failure to act, to their own detriment, however, is insufficient to sustain the cause of action because they did not invoke the authority of the court or cause process to issue. The Dentists’ refusal to properly respond to Leighton’s discovery requests—although sanctionable—is not actionable.”