you have to prove that there has been a lack of the proper formalities, i.e. the rules regarding, for example, how the Will is signed and witnessed
they must know and approve the contents of the Will, and if a challenge is made, then it must be on the grounds that the testator did NOT know and approve the Will
Suspicions might arise if the main beneficiary of the Will is the person who prepared it,
especially if the testator is old or vulnerable, or under the influence of pain killing drugs
Undue influence may be cited, where the testator was coerced into making the Will.
Secondly, if the Will is finally found to be invalid then the previous Will comes into effect.