I read on Yahoo Answers that people who want to be actors should take classes. ?

Chuck Norris was a "martial", not "marshal" artist.

All three men wanted to act (or they would not have become actors). They used what they already learned: how to move and effective timing, how to build up the body and pose, how to sing and tell "down-home-Southern" stories. Then they studied how other actors moved and spoke their lines.

In the days before acting schools, Lillian Gish, the silent screen actress, recounted that before they were famous, she, her sister Dorothy, and Mary Pickford, would go to matinee performances and say, "What discount do you give to fellow professionals?"  Usually they got good seats or could stand in the wings. They watched the actors play their roles. Actors in other touring companies did the same when they saw Mary and the Gish sisters act. They learned from each other. She said they were always learning and trying out how other actors played their roles, how scenes were lit, what would "work" for them.

They also learned from their directors  (Lillian said that Mary would argue with her director D. W. Griffith until he shot the scene twice: her way and his way. Sometimes she won. Not often, and after a Lot of arguing. )

Very, very few actors have ever managed to get paid work with no formal training.  Sure, it's happened for a few but the odds of being taken seriously as an actor, getting an agent, getting auditions, winning a good role, etc, are as likely as winning 50 million on the Lottery if you're not trained.

Acting isn't nearly as easy as some people think.

The competition is HUGE.

For every tiny role in a small production, there will be thousands of aspiring actors ready to fight for it.  No rational director/producer would waste time on an untrained beginner when he/she can choose from maybe 1,000 professionals with years of training and experience.

Typically, an actor would learn these things at acting/drama school.

Speech, diction, learning different accents and dialects, projection, how to vary the tone and range of your voice.

Movement, entering and exiting the stage, how to stand without looking awkward, what to do with your arms and hands.

Most good drama schools also teach singing and dancing (because it improve your vocal abilities and fluidity of movement).

Some also have extra classes in stage combat, various martial arts, circus skills, other languages, stage make-up and hair styling. Some include how to be a stage manager, deputy stage manager, prompter, props manager etc, as many touring theatre companies require their actors to double up in those roles as well.

How to read a script or a play and decide how a character could be portrayed. How to memorise hundreds of lines accurately while not losing the ‘soul’ of the play. How to time your entrances and exits. How to make your lines sound unrehearsed and spontaneous.

The art of acting – there are so many ways to say almost every phrase – which word or words to stress, how to sound natural and realistic while still allowing a live audience to hear you. How to avoid overacting.

They study dozens of different playwrights and their various styles, what messages they were, or are, trying to get across to the audiences. They study the ideas of many of the great acting teachers, such as Stanislavski, Meisner, Chekhov, some schools preferring one approach to another.

How to use facial expressions and gestures to make emotions look real to your audience. How to act without speaking,

How to improvise – necessary for all stage plays and other ‘live’ acting, so that if anyone says the wrong line, forgets a line, enters or exits at the wrong moment, or some disaster happens like a prop breaking or the scenery falling down you can rescue the situation and get the play back on track, ideally without the audience noticing.

You also learn how to get along with everyone, work with people you may not like at all, be able to argue a point without causing offence, when to give in gracefully. How to listen to a director, understand what he or she means and be able to adapt your performance almost instantly as required. They also teach how to adapt what you've learned for when you start acting to camera.

They also cover how to ‘market’ yourself after you’ve graduated, get an agent, how to audition well, how to compile a good résumé (CV), where to get the best headshots, how to get a show-reel and a voice-reel made.

They also advise on how to gain experience after college, by getting together with some other students to put on plays at festivals and competitions, how to hire theatres, how to find open auditions (although there are fewer and fewer all the time).

You would also learn how to deal with rejection, criticism, failure and disappointment without getting depressed, as well as how to deal with success, praise and winning without getting conceited.

Must teach actors to not blink and keep a dolls face before the camera at all times......... 

Quote: "some of the most successful actors have no acting training"

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Even those people have had training. They just received it in a different way than going to a formal acting school. MOST successful actors have had formal training and they'll ALL tell you that it requires hard work to succeed. 

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For every successful actor, there are thousands who aren't. For every actor that can actually support themselves in that profession, there are hundreds who have to support themselves in other ways.

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REALITY CHECK - get one if you can.

Those people were lucky. Not everyone is going to be lucky like that. Everyone who wants to work in a field should learn all they can about it.