Hamlet Speak the speech, I pray you, as I pronounced it to you, trippingly on the tongue: but if you mouth it, as many of your players do, I had as lief the town-crier spoke my lines. Nor do not saw the air too much with your hand, thus, but use all gently; for in the very torrent, tempest, and, as I may say, the whirlwind of passion, you must acquire and beget a temperance that may give it smoothness. O, it offends me to the soul to hear a robustious periwig-pated fellow tear a passion to tatters, to very rags, to split the ears of the groundlings, who for the most part are capable of nothing but inexplicable dumbshows and noise: I would have such a fellow whipped for o'erdoing Termagant; it out-herods Herod: pray you, avoid it.
First Player  I warrant your honour.
Hamlet Be not too tame neither, but let your own discretion be your tutor: suit the action to the word, the word to the action; with this special observance, that you o'erstep not the modesty of nature: for any thing so overdone is from the purpose of playing, whose end, both at the first and now, was and is, to hold, as 'twere, the mirror up to nature; to show virtue her own feature, scorn her own image, and the very age and body of the time his form and pressure. Now this overdone, or come tardy off, though it make the unskilful laugh, cannot but make the judicious grieve; the censure of the which one must in your allowance o'erweigh a whole theatre of others. O, there be players that I have seen play, and heard others praise, and that highly, not to speak it profanely, that, neither having the accent of Christians nor the gait of Christian, pagan, nor man, have so strutted and bellowed that I have thought some of nature's journeymen had made men and not made them well, they imitated humanity so abominably.
First Player I hope we have reformed that indifferently with us, sir.
Hamlet O, reform it altogether. And let those that play   your clowns speak no more than is set down for them; for there be of them that will themselves laugh, to set on some quantity of barren spectators to laugh too; though, in the mean time, some necessary question of the play be then to be considered: that's villainous, and shows a most pitiful ambition in the fool that uses it. Go, make you ready.

Dialogue in Modern English

Hamlet Speak the speech I prepared for you exactly as I told you to, so that it flows smoothly off the tongue. But if you ham it up, as many of your players do, I would just as soon have the town crier speak my lines. Nor should you saw the air too much with your hand, like this [Hamlet gestures with an open hand], but use restraint in all that you do. When the stormy passion of the moment strikes, be temperate and moderate, giving your speech smoothness. O, it offends me to the soul to hear a ranting loud-mouth wearing a powdered wig to tear an emotional passage to tatters, to very rags, to split the ears of the groundlings [persons who paid a penny to stand in the open space in front of the stage], who for the most part are capable of nothing but inexplicable dumbshows [patomiming the action of the play] and noise. [The groundlings–also known as stinklings–were poor lower-class citizens who probably lacked the education and background to fully understand and appreciate a Shakespeare play.] I would have such a fellow whipped for overdoing Termagant; it out-Herods Herod: pray you, avoid it. [Termagant, a Saracen god, and Herod the Great, King of Judea from 37 to 4 B.C., are depicted in plays of Shakespeare’s time as raving tyrants.]
First Player I’ll do as you say, your honor.
Hamlet Don’t be too tame, either. Let your own common sense guide you. Suit the action to the word, the word to the action; but don’t overdo it. Anything overdone strays far from the purpose of acting, which is primarily to hold a mirror up to life to reflect virtue, contempt, and the spirit of the times. Overacting or bad timing–though it might make the unsophisticated laugh–cannot but make the sophisticated grieve. A negative reaction from the judicious, sophisticated theatergoers should be of greater concern than a positive reaction from all the others. O, I have seen players who have been highly praised by others but who cannot rightly be called human beings–not Christian, not pagan, not men at all–because of their strutting and bellowing and their abominable imitation of humanity. I don’t mean to be blasphemous, but these actors seem to have been made by bumbling journeymen assisting God, not God Himself. 
First Player I hope we have reformed such techniques to a certain degree.
Hamlet O, reform them altogether. And let those that play clowns speak no more than is written down for them; for there are some among them that will inject laughter of their own to try to make simple-minded spectators laugh too–right in the middle of an important part of the play. That’s villainous and shows that the fool who uses this technique is contemptibly ambitious. Go now and get ready.