One fundamental rule of evidence with respect to examining your own witness is that you are not allowed to ask leading questions on contentious issues – that is, to suggest answers to the questions that you are asking on matters that are in dispute.
When articulating an objection to a Crown who is improperly leading its own witness on crucial issues, Defence counsel may want to draw upon the following comments from Charron, J.A., speaking for a unanimous Ontario Court of Appeal, in R. v. Rose,  O.J. No. 1150, at para. 9:
A leading question is one that suggests the answer. It is trite law that the party who calls a witness is generally not permitted to ask the witness leading questions. The reason for the rule arises from a concern that the witness, who in many instances favours the party who calls him or her, will readily agree to the suggestions put in the form of a question rather than give his or her own answers to the questions. Of course, the degree of concern that may arise from the use of leading questions will depend on the particular circumstances, and the rule is applied with some flexibility. For example, leading questions are routinely asked to elicit a witness' evidence on preliminary and non-contentious matters. This practice is adopted for the sake of expediency and generally gives rise to no concern. Leading questions are also permitted to the extent that they are necessary to direct the witness to a particular matter or field of inquiry. Apart from these specific examples, the trial judge has a general discretion to allow leading questions whenever it is considered necessary in the interests of justice: Reference Re R. v. Coffin (1956), 114 C.C.C. 1 at 22 (S.C.C.).
When the state’s examination of its own witness on contested issues starts to sound like a leading cross-examination, Defence counsel has a duty to object promptly.
The state is not permitted to prove its case by putting words in the mouth of its own witness at a criminal trial.