One of the components of testamentary capacity and a valid will is that the testator must have knowledge and approval of the contents of the will before it is signed.
In his book Capacity and Undue Influence (Toronto: Carswell, 2014), John E.S. Poyser discusses the distinction at page 235:
Lord Justice Moore-Bick went on to comment on the distinction between testamentary capacity, on the one hand, and knowledge and approval on the other, giving an elegant formulation to distinguish between them (emphasis added):
The use of the expression “knowledge and approval” is liable to give the impression that the court is concerned with whether at the time he executed the will the testator must be able to reconsider all the dispositions he has made. That would require testamentary capacity, but that is not what is meant by the convenient expression “knowledge and approval”. Modern authorities recognise that a clear distinction is to be drawn between testamentary capacity and knowledge and approval. As the judge observed in this case
….testamentary capacity includes the ability to make choices, whereas knowledge and approval requires no more than the ability to understand and approve the choices that have already been made.
Paraphrasing a different comment elsewhere in the reasons for decision, the twin requirements of knowledge and approval in testamentary capacity ensure that the will is the product of the conscious intention of a sound mind. Knowledge and approval is the “conscious intention” in that formula.
“In my view it is clear from decisions of high authority that where the propounder of the will is shown to have participated in the drawing and preparation of the will and to take a benefit under the will, he must satisfy the conscience of the court that the part of the will under which he benefits had the full knowledge and approval of the testatrix. I refer to Fulton v. Andrews (1875) L.R. 7 HL 448, Wintle v. Nye,  1 W.L.R. 284,  1 All E.R. 552 (both decisions of the House of Lords). Fulton v. Andrews was applied in Re Timlick; Timlick v. Crawford, supra, and by the Supreme Court of Canada in Re Martin; MacGregor v. Ryan, supra. Other decisions have also applied Fulton v. Andrews and Wintle v. Nye.”
The will-maker must be aware of the magnitude of the residue of her estate and must appreciate the effect of the disposition of her estate .
Faulkner v Faulkner 1920 SCC 386 seems to suggest that the test for knowledge and approval is perhaps not very high.
A lawyer prepared a will as instructed by the testator, who was fully competent when giving instructions, but when the will, so drawn was presented for execution, the will maker was not in a condition to sign his name and refused to execute it as a marksman. Three days later on the day he died, it was again presented and read over to them, clause by clause, by the lawyer who as each was read asked if he understood it, and he indicated that he did. The will was then executed by the testator, making his mark on the will with the lawyer guiding his hand, as the testator could not see.
The court held that the evidence of the lawyer and of the physician in attendance to establish that the mental capacity of the testator to follow the reading of the will, and to realize that his instructions had been carried out, had been met.
The court cited the decision of the privy Council Perera v Perera (1901) AC 354 , that stated the law, as follows:
“If a person has given instructions to a solicitor to make a will, and the solicitor prepares in accordance with those instructions, all that is necessary to make it a good will, if executed by the testator, is that he or she should be able to think thus far – I gave my solicitor instructions to prepare will making a certain disposition of my property; I have no doubt that the lawyer has given effect to my intention, and I accept the document which is put before me, as carrying it out.”