What must the victim apprehend?
The victim must apprehend immediate unlawful force, meaning they must expect or anticipate that they are about to be subjected to violence. Apprehension refers to the expectation of force rather than fear, as recognised in R v Nelson.
What happens if the victim knows that the defendant cannot actually carry out the threat?
If the victim knows that the defendant cannot actually carry out the threat, then there will be no assault because the victim cannot apprehend force. This was demonstrated in R v Lamb, where the victim knew the gun was not loaded. Nevertheless, the force does not actually need to be possible if the victim believes it could occur. In R v Logdon the defendant pointed a fake gun at the victim and said it was real, which was sufficient for assault because the victim believed the threat was genuine.