Regarding the program you are referring to, I would say that a large number of the examples seen there are inspired by what is done in civil cases in the United States. In Quebec and Canada, there are very few juries in civil matters, and in criminal matters, there are some quite specific guidelines for jurors, and so the American examples likely apply less. You could summarize by saying that it is in large measure the lawyer or prosecutor's instinct that is the determining factor. Mr. Lévesque was talking about a person who looks at his feet while talking. That is the sort of factor that is taken into account.
As for peremptory challenges, a jury trial may be an imperfect process. Non-lawyers will have to decide on the substance of a case, on a charge that may determine a person's freedom, and that is why it is important to ensure that the accused and society accept the legitimacy of the jury and of the ultimate verdict.