Don John appears in one scene in Act III -- scene ii. It is important to note that, earlier in the play, Don John tells the audience that he plans to bring down his half brother Don Pedro's friend, Claudio. Don John calls himself a "plain dealing villain" (I, iii, 30), and so he operates as the villain of this comedy, creating complications for the other characters to untangle.
It is also in Act I, scene iii that Don John gives his reason for villainy. He nurses a vague sort of hatred towards Claudio, giving his rationale in lines 61-63:
. . .that young start-up [Claudio] hath all the glory of my overthrow. If I can cross him any way, I bless myself every way.
These remarks about his jealousy over Claudio's promotion within the military is reminiscent of the jealousy harboured by Iago in Othello, which comes, at that play's conclusion, to a much more tragic resolution.
So, in Act III, scene ii of Much Ado, Don John is preparing both his half brother and Claudio to witness a charade he has devised to "prove" to Claudio that Hero (whom he intends to marry) is "disloyal." He invites the two to stand beneath her window and witness her being unfaithful. There isn't much to analyze here, as Don John is direct and pointed in his accusation, and Don Pedro and Claudio are more than willing to follow him.
I would say that, in this scene, Don John is bold and confident in his self-assurance. He accuses Hero in no uncertain terms, saying:
The word [disloyal] is too good to paint out her wickedness. I could say she were worse; think you of a worse title and I will fit her to it.
And Claudio and Don Pedro accept this without protest or question. Don John has, ultimately, quite an easy job to lead the two gullible men to the set-up he has devised under Hero's window, and they fall for his ruse hook, line and sinker, as the audience is told by Borachio in the very next scene.
For more on Don John and Act III, scene ii, please follow the links below.