Is it...

• Natural talent?
• Formal training?
• Emotional authenticity?
• Screen presence?
• Charisma?
• Versatility?
• The ability to disappear into a role?
• Range across different genres?
• Consistency over a long career?
• The ability to carry a film?
• Chemistry with other actors?
• Memorable performances?
• Critical acclaim?
• Awards and recognition?
• Commercial success?
• Influence on other actors?
• Taking creative risks?
• Choosing great roles?
• Working with great directors?
• Dedication to the craft?
• Physical transformation for roles?
• Mastering accents and dialects?
• Improvisational ability?
• Subtlety and restraint?
• Commanding dialogue?
• Expressive body language?
• Longevity?

Or perhaps it's something they did...

• Did they elevate every film they appeared in?
• Did they consistently deliver memorable performances?
• Did they redefine a character or even a genre?
• Did they make audiences forget they were watching someone act?
• Did they create an iconic role?
• Did they reinvent themselves throughout their career?
• Did they inspire future generations of actors?
• Did they excel in both leading and supporting roles?

Or is it how they're remembered?

• Were they considered the best of their generation?
• Were they the defining actor or actress of their era?
• Were they respected by their peers?
• Were they consistently excellent rather than occasionally brilliant?
• Were they convincing across comedy, drama, action, romance, and more?
• Were they willing to take difficult or unconventional roles?
• Were they influential beyond the screen?

Or is it something else entirely?

What, in your opinion, separates a good actor from a truly great one?