Don't act. React. Don't say lines. Speak concepts. Don't take too long. Get the words out clearly and as quickly as possible using pauses rarely and only for dramatic effect, but don't rush either.
Don't push your voice down to sound 'macho'; you'll only sound stupid. Don't speak with an elevated pitch to sound excited, you'll sound even worse. Just relax and let your voice flow as you would in daily conversation. In other words, don't even think about it. Just do it. If you try to sound professional, you'll sound like you're trying to sound professional. Be you. If you're not professional, then work that out in real life, not on the set or in rehearsal. Adopt the attitude of your character before walking on to the set. Maintain their attitude the entire time you are in character.
Understand the character you are playing. Discern his motives or why the writer has your character reacting the way he does to other stimuli in the movie. Understand the thoughts of people like your character by studying real life - what are the thoughts, actions, idiosyncrasies, facial expressions, body language, and values of real life people similar to your character? How would they react? Remember, it's better to not prepare at all, than to halfway prepare, ... or worse yet to work on your technique in the middle of your audition or as you are performing your role on the set. Over-memorize everything, but don't feel that you have to deliver it word-for-word unless the director insists. Just deliver the key concepts and words which are needed. The words flowing out of your mouth should be the last thing on your mind.
Ever see an actor struggling with his lines mentally which everyone who is watching his face can tell? Pitiful is the actor who appears to be thinking about his lines for an audition. Another faux-pas is when an actor acts like a high school stage actor on the set of a movie. Stage acting is far different than film acting. Stage actors exaggerate movement, volume of speech, and emotion for an audience that may be seated far in the back of the theater. Such exaggeration looks totally ridiculous on a movie set unless the purpose of the movie is to look ridiculous (such as in a comedy). Movie acting is more about being one's self.
I've successfully used non-actors for movie roles, simply because if you find someone who already acts like the script character in real life, they only have to be themselves in front of the camera. As long as they don't hyper-ventilate or suddenly start "acting" and just remain calm and act normal, they usually look pretty great in a movie role. Imagine using a real-life CEO from New York to act the part of an NYC Chief Executive Officer in a movie; as opposed to using an actor whose real-life job is that of a fast food server in the deep South. Who can pull it off better? Unless you have an amazingly talented individual, I'd prefer to audition some real-life CEOs who are non-actors than trained actors who lack the polish, acumen, speech, posture, and understanding that a real-life CEO has to offer.
Obviously, the ultimate actor is someone who is trained in movie acting and is a real-life CEO (or has worked in that capacity in the past), but such an actor is not always readily available. Regarding stage actors, it is sometimes extremely difficult to get an actor who has been trained in theater stage acting to look realistic in front of a camera, because they are so trained to exaggerate their action, volume, facial expressions and such. They often just look overly dramatic.
Preparation starts far in advance of any role being allocated to them. Actors must speak everyday with proper speech, intonation, and interpretation and know how to emphasize words, instead of trying to transform themselves into a great speaker on set. Be a great speaker in real life. Understand grammar, language, international phonetics, pronunciation of vowels, and which words to emphasize when you're practicing, but don't bring those technical thoughts on to a set. It should already be a part of your life.
If you speak with improper grammar and speech in real life, then just roll with that - don't try to sound like someone you're not. Or take the time to learn proper grammar and speech far in advance of any pursuit of a career in acting. Be eloquent. Every vowel must be proper. Every consonant must drop into place. Every nuance of sound must be used to emphasize some words above others in order to interpret a phrase properly. Punctuation must be used appropriately at the end of phrases or sentences. Typically, a period at the end of a sentence has a longer pause than a comma at the end of a phrase, while a question mark is usually voiced with a slightly higher pitch at the end of a sentence than a period.
An acting professor at University of North Carolina once told me that singers usually are the better actors - perhaps it is because they understand the melodic nature of pitch in speaking and its nuances? Singers also understand such concepts as diaphragmatic breathing, pronunciation, resonance, pharyngeal space, dynamics, and posture.
If you have to sound as though you have a particular geographical or ethnical accent for a part, then learn that language (Chinese, French, Russian, etc) or immerse yourself in recordings of their dialect (British, Irish, Scottish, Welsh, Australian, high English, low English, cockney, Southern country, etc) and discern the rules of pronunciation that guide that particular language or accent from the standpoint of international phonetics. Also, if you are to speak English with a Russian accent, spend time speaking with proper Russian for several minutes before you walk on to the set. It will condition your tongue and teeth to sound like you have a Russian accent.
Let me emphasize this again. Don't act. React.
To what are you reacting? Do you know? Don't attempt to display emotion. Let it flow as a byproduct from your reaction towards whatever is happening on the set. Your words and actions are just side effects. What you are trying to communicate to someone is your sole focus. Some poor actors think that crying, yelling, or acting emotional is 'good acting'. I'd say it's pathetic acting. If you cry, yell, or get upset as a result of reacting, that's great; however, only an inexperienced and untalented actor would make that their goal as an actor or think that generating emotional behavior is acting.
Keep your eyes glued on to your focus on the set. Absolutely never take your eyes off that object of focus to glance around. Never let your eyes wander. Keep them trained on the thing to which you are reacting. One technique that may help you with understanding your emotions in a scene is to close your eyes, so that you block out all distractions. Then speak through your lines in the dark. This can help you focus in on the emotion of a scene or moment. You'd be amazed at how practicing with your eyes shut will help you in your acting.
Please don't just speak memorized lines but rather respond to the external actions or words or stimuli that precede your own actions or words. Understand motives, study character profiles, memorize micro-expressions and body language, practice miming, say your lines in the dark or with your eyes closed, but when you step on to the set, don't think about acting - just react.
It's like swimming. Just jump into the water and start swimming. Swimmers may think through strokes, technique, and breathing far in advance of a match, but when an Olympic swimmer jumps into the pool, technique is the last thing in their mind - that's already conquered - they are now thinking about reaching the other side of the pool swiftly and all their energy and thoughts are at work in achieving this goal.
Again, it's like singing. If you look like you're trying to remember the lyrics while working on diaphragmatic breathing or your pronunciation when you are in front of the audience, then you will look ridiculous. A great singer has that all polished far in advance and when they step on stage. They just focus on the object of their reaction and let their words roll off their tongue and allow their emotions to rise within them ... naturally, not forced. They communicate the words to the audience with their every thought embracing the meaning of each word and their face manifesting the emotions of every word they are saying. Put on your character as though it's a familiar coat and simply live the role.
React. Over-prepare, but forget technique and live in the meaning and emotion of what you are communicating as you react to whatever happens on set.