The phrases "You will not pass" and "You shall not pass" are often used interchangeably, but there are subtle differences in their connotations and usage:

1. "You will not pass":
   - This phrase is more straightforward and is often used to indicate a future event that will not happen. It suggests a prediction or a strong likelihood that the subject will not succeed in passing.
   - It is more informal and can be used in various contexts.
2. "You shall not pass":
   - This phrase has a more authoritative or formal tone. The word "shall" often implies a stronger obligation or command, suggesting that the speaker has the power to enforce this prohibition.
   - It is famously associated with Gandalf's line in J.R.R. Tolkien's The Lord of the Rings, where it conveys a sense of moral or ethical authority.

In summary, while both phrases convey the idea of preventing passage, "you shall not pass" carries a stronger, more commanding tone, whereas "you will not pass" is more about stating a future outcome.