Astanga Hrudaya
धीरः स्वरोऽनुनादी च, वर्णः स्निग्धः स्थिरप्रभः। स्वभाजं स्थिरं सत्त्वमविकारि विपत्स्वपि॥११४॥
the voice should be courageous (loud, commanding attention) and vibrating. The colour (of the skin) should be unctuous / greasy and with good lustre the mind (mental activities) should be natural, firm and not undergoing change even at times of danger (remaining steadfast even in trying situations)
english translation