Diction is essentially your choice of words and how you speak or write them. In Shakespearean performance, excellent diction means staying true to the text as it was written, and delivering the lines in a manner that is consistent with the meaning and organization of your character and the rest of the play. Much of the enjoyment an audience gets from great Shakespeare is the result of excellent diction, while much of the confusion and despair that comes from an inability to understand the action of a Shakespearean play comes from actors paying too little attention to the form and structure of what they are saying.
Shakespeare employed three basic forms of versification in his plays. Each uses distinctly different diction and requires distinctly different approaches in performance.
The bulk of Shakespearean lines are written in the format known as Blank Verse. Quite simply this means that the lines do not rhyme. Each line is usually comprised of words that have a combined total of about ten syllables, or five pairs of syllables, each of which is known as an Iambic foot. Since there are five Iambic feet per line the line structure is known as Iambic Pentameter.
In performance the syllables in an iambic foot are usually spoken with the emphasis placed on the second syllable, in what is known as an unstressed-stressed format. This means that a line like "Two Households, both alike in dignity" would be spoken with a slightly stronger emphasis on the syllables House, both, like, dig and ty.
Understanding this structure is the key to a successful delivery. While not all of Shakespeare's blank verse is exactly ten syllables per line or stressed in exactly this way (if it were it would quickly become boring) being able to see the structure of a line and using that structure when speaking it makes the line sound the way Shakespeare intended it. Thus, failing to memorize even one word can throw of your delivery of an entire line, and putting the emphasis on the wrong syllable can make the line sound confusing to even an experienced Shakespearean scholar sitting in the audience.
Maintaining excellent diction in the second of Shakespeare's preferred writing forms is even more important. For working-class characters like the fighting servants at the beginning of Romeo and Juliet or the Mechanicals in A Midsummer Night's Dream Shakespeare chose to distinguish them from the nobility by writing in prose, with no line structure whatsoever.
When performing Shakespearean prose it's important not to slip into the unstressed-stressed form of delivery of the blank verse that often surrounds it, but instead to perform it as close to everyday speech as possible. This often results in a distinctive change of pace for the audience. Again though, it's important to know all the words, as quite often Shakespeare's lower-class characters make extensive use of the slang terms and colloquial references that are foreign to us today, and usually come with an explanatory footnote at the bottom of the page. In performance the audience have no footnotes to rely on, so it's up to the actor, in his or her delivery of the lines, to make their meaning clear.
The final major Shakespearean verse form is the rhyming couplet. Shakespeare often uses a couplet to end a long speech, or as a passage of several couplets strung together. Sometimes one character begins a couplet, only to have another one end it, often to comedic or romantic effect.
When performing rhyming couplets it is equally important to remember all the words in the line, not just the ones at the end which form the rhyme. If one half of the line is significantly longer or shorter than the other chances are the audience will miss the completion of the thought, as the first line of the couplet has begun an idea in a given length of time, so the brain expects the second line to complete that idea in about the same number of seconds. If that doesn't happen quite often the meaning will be lost.
Excellent diction is essential to the understanding of Shakespeare in performance, especially for the actor. A performance where the lines are complete, clear and properly arranged will make the play easy to watch, listen to and enjoy, even for young people or those with no prior knowledge of Shakespearean works. A good actor will take the time to learn and memorize the structure of his or her lines, and learn to deliver them as they meant to be delivered, in addition to the usual processes of acting like creating a character and finding playable truths within the text. Only when their acting skills are combined with an impeccable delivery will their efforts result in an unforgettable performance.