There are two possible outcomes with respect to Friar Laurence's plan to fake Juliet's death detailed in Act 4, Scene 1. The first possible outcome is expressed in Juliet's speech just before she drinks the vial. Just like she, the reader/viewer can predict that the vial may not fake her death so much as cause her actual death. A second possible outcome the reader can predict is that Romeo may think her death is actually a real death, which could lead to other travesties. All in all, the reader/viewer sees that things can possibly go wrong with this plan, and it creates a great deal of suspense.
While Juliet, in her speech before drinking the vial, never worries that Romeo may think she is really dead, she does express the fear that the vial might really kill her. We see her worrying the vial actually contains a poison meant to kill her so that Friar Laurence will not have to be guilty of marrying her to two men, as we see in her lines:
What if it be a poison which the friar
Subtly hath ministr'd to have me dead,
Lest in this marriage he should be dishonour'd
Because he married me before to Romeo? (IV.iii.25-28)
While Juliet does not herself consider the possibility that Romeo may not learn the truth in time, she does picture in horror what could happen if she had to wait for Romeo in the tomb. She pictures herself suffocating in the vault before he comes or being driven mad by her surroundings to the point that she kills herself. Regardless, all in all, the reader/viewer knows that many consequences can result from Friar Laurence's plan, two of which are that she could actually die, or Romeo could think she is really dead. In addition, the possibility that anything can go wrong with the plan helps create suspense.
The feeling of suspense is created both by what Juliet will experience by taking the friar's potion and what could go wrong. First of all, the best case scenario is that she will awake in her family's tomb, sharing the space with her ancestor's rotting bodies. (She speaks of playing with their bones.) That's spooky enough, but think of all the ways (and Juliet does) that the plan could go wrong. She could take a wrong dose of the potion -- she considers that the friar might actually be trying to kill her because of his guilt in secretly marrying them-- and actually die. Or somehow, the message to Romeo is lost and she would be buried alive in the tomb. Either way, her prospects look pretty grim.