Intermissions as modern audiences conceive of them were unknown in Shakespeare's time. Plays were either performed continuously in the public theatres, or in the private theatres were broken up by brief musical interludes. This fashion of breaking up the play with musical interludes was adopted by the public theatres in about 1610, but no audience of the time would have recognized the modern convention of the single 15 (or dual 10) minute break (94 - 95).
Imposing the modern convention on the early modern text will disrupt the flow of energy. Even when an interval comes after a climactic moment and the action resumes in a powerful way (as is the case when the interval is placed between 3.3 and 4.1 of Julius Caesar (Cinna the Poet and Proscription), the link in the dramatic action between the violent death of the plebeian and the machinations of the triumvirs is diminished (95).
Some modern productions will play through without an interval, and Dessen provides a list of examples, and locations where the convention is accepted here (95 - 96).
The interval can be used as a moment to either introduce some new action into the world of the play or to remove large and/or complicated set pieces. Dessen cites Orlando "regularly" seen by audiences posting love notes during the intermission, along with other more specific examples (98).
"As with so many other theatrical choices in interpreting Shakespeare, to raise such a question (where should the interval/intermission come in The Tempest?) is to call attention to the many options in how we respond to, value, or trust the signals or strategies in the original scripts" (99).
It is important to recall that Shakespeare wrote plays for audiences and conventions that died long ago, and an intermission is an expected piece of modern theatrical conventions that is not easily dispensed with. Truly great directors will find a way to insert this break in the action in a way that works to their advantage to highlight a specific moment, or to create stage business that tells another part of the story, and to start again strong (108)
My standard rule of thumb is that an audience will remain seated for about 90 minutes, and thus a play that is about 90 minutes or shorter doesn't need an intermission. Merry Devil was only 88 minutes on a bad day, so there was no question of inserting one, although we did have plenty of opportunities, and I am told that Erin Baird, an alumna of my program, wrote an excellent thesis on certain indicators of musical interludes which I ought to look into. The action of Merry Devil is broken because the text is broken, and thus the play would easily accommodate an intermission if it were anywhere long enough to hold one.