Birdboot falls for Cynthia
On the heels of telling Moon that he is nothing but faithful to his wife, Myrtle, Birdboot becomes smitten with the actress playing Cynthia. Despite taking the actress playing Felicity out for a drink the evening before (and vehemently denying their date), Birdboot decides Cynthia is now worthy of his critic's seal of approval. This is a bit of comic irony that helps to illustrate the chasm between what the characters say, and how they feel. It also exposes Birdboot for the philanderer Moon accuses him to be.
Puckeridge the murderer
Moon's greatest fear is that he will never transcend the shadow cast by Higgs' first-string status. His identity is determined by Higgs' absence, and he ceases to exist in Higgs' presence. He dreams of a time when the understudies kill the leads. It dawns on Moon that Puckeridge, the third-string critic behind him, may want him dead just as he fantasizes about Higgs' demise. Thus, when Puckeridge is revealed to be the actual murderer - not just in the play-within-the-play, but as the killer of Moon, Birdboot and Higgs - Moon's fear comes to pass. The reveal is a twist Stoppard employs to highlight the clash of realities at the end of the play.
Birdboot and Simon Gascoyne
Birdboot and Simon Gascoyne share a similar wandering eye. As Simon is revealed to have romanced both Felicity and Cynthia, Birdboot's actions give away his own similar entanglements with the actresses playing those roles. He is unable to contain himself when he sees Cynthia, and his own performance of a dutiful husband falls away. When Birdboot becomes part of the play, he assumes Simon's role and is able to insert his own truthful affections seamlessly into the play's dialogue. When he gets up on stage, Birdboot trades one performance for another, and is killed for it.
Simon and Hound the critics
Moon rushes to Birdboot when he is shot. Dismayed at his death, he attempts to retake his seat in the critics' row only to see that Simon and Hound have usurped him. Simon and Hound give their review - and it is quite unfavorable. They say it lacks "pace, point, focus, interest, drama, wit [and] originality". This is a complete counterpoint to the glowing review Moon and Birdboot rattle off earlier in the play, seemingly without actually watching the hackneyed portrayal of Muldoon Manor. Simon and Hound's review is more accurate to the play they were just in, where the real drama Moon is entangled in is quite high stakes - a man has actually died onstage in front of the audience.
