1. First, Shakespeare wrote his plays in blank verse featuring iambic
pentameter because that was the style of the day. Think of it as a way
for an author to show off–and it really is quite impressive if one
thinks about it. There are very few authors who can create characters
and plots as rich as Shakespeare’s and write their lines in a consistent
meter.
2. Secondly (I think that this might be what you are asking), when
Shakespeare’s characters speak in verse (iambic pentameter), they are
usually the noble (aristocratic) characters, and their speech represents
their high culture and position in society. If you simply look at one
of Shakespeare’s plays, you can often tell when the commoners are
speaking because their lines will go from margin to margin (this is
true, too, of nobles who are acting like commoners–whether they’re
involved in evil schemes, losing their minds, or are drunk!). In
contrast, Shakespeare’s other characters’ lines should sound and look
different to you–they should sound “sing-songy” and should look like
poetry with uneven lengths.
A good example of this is from Othello. When Iago is speaking to his
peers or to those in position of authority over them, his speech is in
verse, but when he is plotting and talking to Roderigo (especially at
the play’s beginning), his lines are not in iambic pentameter–this
represents the bawdy nature of his speech and, in truth, the baseness of
his character.