Like many of Pinter's other plays, very little of the expository information in The Birthday Party is verifiable; it is often contradicted by the characters and otherwise ambiguous, and, therefore, one cannot take what they say at face value. For example, in Act One, Stanley describes his career, saying "I've played the piano all over the world," reduces that immediately to "All over the country," and then, after a "pause", undercuts both hyperbolic self-representations in stating "I once gave a concert."
While the title and the dialogue refer to Meg's planning a party to celebrate Stanley's birthday: "It's your birthday, Stan. I was going to keep it a secret until tonight," even that "fact" is dubious, as Stanley denies that it is his birthday: "This isn't my birthday, Meg" (48), telling Goldberg and McCann: "Anyway, this isn't my birthday. [...] No, it's not until next month," adding, in response to McCann's saying "Not according to the lady [Meg]," "Her? She's crazy. Round the bend" (53).
Although Meg claims that her house is a "boarding house," her husband, Petey, who was confronted by "two men" who "wanted to know if we could put them up for a couple of nights" is surprised that Meg already has "got a room ready" (23), and, Stanley (being the only supposed boarder), also responds to what appears to him to be the sudden appearance of Goldberg and McCann as prospective guests on a supposed "short holiday," flat out denies that it is a boarding house: "This is a ridiculous house to pick on. [...] Because it's not a boarding house. It never was" (53).
McCann claims to have no knowledge of Stanley or Maidenhead when Stanley asks him "Ever been anywhere near Maidenhead? [...] There's a Fuller's teashop. I used to have my tea there. [...] and a Boots Library. I seem to connect you with the High Street. [...] A charming town, don't you think? [...] A quiet, thriving community. I was born and brought up there. I lived well away from the main road" (51); yet Goldberg later names both businesses that Stanley used to frequent connecting Goldberg and possibly also McCann to Maidenhead: "A little Austin, tea in Fuller's a library book from Boots, and I'm satisfied" (70). Of course, both Stanley and Goldberg could just be inventing these apparent "reminiscences" as they both appear to have invented other details about their lives earlier, and here Goldberg could conveniently be lifting details from Stanley's earlier own mention of them, which he has heard; as Merritt observes, the factual basis for such apparent correspondences in the dialogue uttered by Pinter's characters remains ambiguous and subject to multiple interpretations.
Shifting identities (cf. "the theme of identity") makes the past ambiguous: Goldberg is called "Nat," but in his stories of the past he says that he was called "Simey" (73) and also "Benny" (92), and he refers to McCann as both "Dermot" (in talking to Petey ) and "Seamus" (in talking to McCann ). Given such contradictions, these characters' actual names and thus identities remain unclear. According to John Russell Brown (94), "Falsehoods are important for Pinter's dialogue, not least when they can be detected only by careful reference from one scene to another.... Some of the more blatant lies are so casually delivered that the audience is encouraged to look for more than is going to be disclosed. This is a part of Pinter's two-pronged tactic of awakening the audience's desire for verification and repeatedly disappointing this desire" (Brown 94).
Although Stanley, just before the lights go out during the birthday party, "begins to strangle Meg (78), she has no memory of that the next morning, quite possibly because she had drunk too much and gotten tipsy (71–74); oblivious to the fact that Goldberg and McCann have removed Stanley from the house – Petey keeps that information from her when she inquires, "Is he still in bed?" by answering "Yes, he's ... still asleep"––she ends the play focusing on herself and romanticising her role in the party, "I was the belle of the ball. [...] I know I was" (102).