Juliet expresses one of her fears when she says, “What if this mixture do not work at all? Shall I be married then to-morrow morning?” (IV.iii.21-22).
We can tell that Juliet is worried that Friar Laurence had given her poison when she says “What if it be a poison, which the friar subtly hath minister’d to have be dead” (IV.iii.24-25)
We can see how nervous Juliet is about waking up before Romeo gets there when she says, “[What if] I wake before the time that Romeo come[s] to redeem me? There’s a fearful point! Shall I not then be stifled in the vault” (IV.iii.31-32).