Do actors have to train for as many hours a day as musicians or dancers?

Question

If so, what kind of things do they do during all those hours?

Answer 1

Yes. They train long and hard to be able to "act" real. It doesn't always come naturally. Taking acting classes helps them to be able to learn the techniques to creating characters who are believable.
Actors often work with a drama coach. They research their roles so they can understand the story's setting and background. Sometimes they learn a foreign language or train with a coach to develop a certain accent to make their characters realistic.
Actors need a lot of talent in order to play different parts. Skills such as singing, dancing, skating, or juggling may be important. Actors must have self control and be able to follow directions.

Answer 2

I go to theatre production and design school and I work a lot with actors. Basiclly, training is a MUST. It's very important to get training from a well respected educator.

What they do is learn not only to control their voice but their entire bodies and emotions.

Trust me, I done some of the emotional stuff (its called the Method), if you are not propared, it can certainly screw you up.