Most actors can make the 'first cut' and are worth mentoring to the next level if they have the following basic qualities.
Primary Positive Attributes
Unique, special or great looks
Unique, special or great personality
Ability to emote on queue
Great passion for the craft
Teachable and humble
When you audition an actor, you must think of it as auditioning their speech, body, mind, eyes, and heartfelt emotions. The script is only a tool and should be used to allow the actor to demonstrate their abilities in each of these areas. Some think that acting is showing how they can scream or cry, but acting takes more than such emotive qualities. Each area should be assessed apart and also as a whole.
The primary skills take longer and must be conquered first. Nasal and indistinguishable speech is not acceptable to any director. Realizing that you sound this way may not be obvious to you without guidance and a mentor. Proper speech technique, basic understanding of language and speech such as the international phonetic alphabet (IPA), and phonetical soundings of each consonant and vowel are helpful. Understanding the physiology and mechanics of the voice are very important such as pharyngeal space, resonance, and diaphragmatic breathing.
An actress, such as Grace Kelly, isn't an accident. It takes work. Being physically fit and having proper posture, such as walking with a book on the head, are nice to know. The art of pretending such as being able to suddenly go into character and emote tears, anger, or joy is a gift that actors will use again and again. A passionate love for the craft of acting and moviemaking, as well as a positive, humble attitude are absolutely mandatory.
Secondary Negative Attributes
Some secondary qualities that also need work, and which can be very annoying to the director and to the audience include:
Awkward hands - inappropriate or contrived hand motions
Forced emotion - physically generated instead of mentally
'Speechy' - sounds like one is reading, not conversing
Actor's eyes - lack focus on the one across from them
Mental fraud - not comprehending one's lines or feeling them
Lack of internal reactions - acts mechanical (not natural)
Lack of proper micro-facial expressions or body language
Failure to study a role mentally (i.e. interviews, books, etc)
Failure to mimic their character physically (i.e. martial arts, etc)
Once primary skills are conquered, the focus can move toward the secondary qualities mentioned above such as practicing lines in front of their cellphone camera while addressing a photograph of a friend or loved one. They should then view the video and critique themselves on each of the secondary skills above.
After they feel they are ready, they should then find a professional, even if they have to pay them, who can critique their performance, word by word, reaction by reaction. Many video tutorials address micro-facial expressions and body language which you can view online. Another one of our blog articles offers a quick look at some of these communication skills.
Studying one's role or character is important to ensure an actor understands the mentality of their character, have interviewed similar characters in real life (if their character is fictional or not alive still), and can perform the same physical stunts, mannerisms, or physical activities such as martial arts or fencing or archery as their assigned character.
These are a few of my favorite attributes and pet peeves in professional actors. Hope it helps you!