Just about everybody speaks in unrhymed verse called blank verse. Some minor characters never do, and many characters switch to ordinary prose from time to time, but most of them use blank verse as a rule.
Prose. But upper-class folks often speak in prose also, especially if they are losing their grip on things. Lower class characters often appeared in plays as comic relief or comic foils to the upper-class ones, and so the more conversational and less formal-sounding prose rhythms suited them.
People of common origin or who are comedic often speak in prose; those who are noble or are expressing noble sentiments are more likely to speak blank verse.
Comedic or lower-class characters spoke in prose; the rest spoke in verse.
the commoners
brave and powerful ones
Why did Shakespeare use iambic pentameter for the dialogue of noble characters
He has the characters in the play say them. That is how you use words in a play.
Hamlet! (A little more than kin and less than kind)
Shakespeare's diction was blank verse, rhyme and prose.
Generally (but not always!) Shakespeare's characters who spoke in blank verse are the lower-status characters. Think of which characters are not as important, then compare that to some of their speech in Romeo and Juliet.
Why did Shakespeare use iambic pentameter for the dialogue of noble characters
No, as witness the astounding variety of characters he created. If blank verse did not suit the character, Shakespeare just didn't use it.
He has the characters in the play say them. That is how you use words in a play.
Hamlet! (A little more than kin and less than kind)
Shakespeare's diction was blank verse, rhyme and prose.
underground
The contrast between the appearance and the reality of the characters gives Shakespeare's characters depth.
Shakespeare's heroines were his female characters.
Generally (but not always!) Shakespeare's characters who spoke in blank verse are the lower-status characters. Think of which characters are not as important, then compare that to some of their speech in Romeo and Juliet.
Shakespeare did not write a work called "The Banquet".
Shakespeare used masks in his plays to give the characters mystery, or decite. Also to make the characters easier to follow, like which in Midsummer Nights Dream, he used masks to show who was the animals.
Actually Shakespeare did not "write about" any characters at all, except when other characters are talking about them. Shakespeare created his characters by writing words for them to say and actions for them to do. He also created an awful lot of characters; if you pick up a copy of any Shakespeare play whatsoever, and look at the beginning where it lists the characters in that play (the Dramatis Personae), you will see the names of more than seven characters, guaranteed. Twelfth Night, a comedy, has fourteen characters, Macbeth, a tragedy, has about 28, the First Part of Henry VI, a history, has 37. Another hint: the names of 23 of Shakespeare's characters appear in the titles of his plays.