Shakespeare employs masks in different ways in Much ado about nothing.
In II.1 the theme of disguise and mistaken identity is given prominence. During this scene the theme of being masked is employed as a common entertainment in Elizabethan times as well as being misemployed in order to be up to one's mischief as done by Don Jon and his fellow Borachio.
In II.2 the paramount importance of disguise is still present in their scheme, however the tenor is more serious.
In scene two of Act III the link to appearance masking character is established by the image of fashion. Benedick says very little in this scene compared two others marking his changed character, while Claudio and Don Pedro talk about him and his new outward appearance.
The theme of fashion is sharply in focus in scene III.4 again, together with the theme of illness. The concept of disguise is highlighted by this scene. Once it is stressed by the idea of fine clothes masking true feelings, as well as by pretending to feel ill in order to hide them from others.
Scene one of Act IV is written in blank verse to stress the importance of its content. Additionally, this scene is eye-catching because of the frequent references to disguise and masking the truth in Claudio's accusatory speech.
The tide is turned in scene one of Act V and again the thread of disguise is obviously present as Claudio expected to learn a hard lesson before he is marrying Hero. This case is quite interesting as the plot has been invented by the Friar himself, which shows that hiding the truth has been quite common.
This very special plot is continued to the end of the play (V.4).
At this point the circle is closed and real masks are used to hide again. The difference compared to the former usage of masks is, that it is purely positive throughout the whole scene, without the intention to harm somebody.