Short answer. The court must declare it, and only on an adequate showing of one of three things: the witness's adverse interest, his unjustified reluctance to testify, or his having misled the party into calling him to the witness stand. Disappointing testimony alone is not one of them.
What the law says
A witness may be considered as unwilling or hostile only if so declared by the court upon adequate showing of his or her adverse interest, unjustified reluctance to testify, or his or her having misled the party into calling him or her to the witness stand.
Rule 132, Section 13 — Party may not impeach his or her own witness. Read the full provision →
It is the court's declaration, not the party's label
The word only carries most of the weight in this sentence. A witness may be considered unwilling or hostile only if so declared by the court. A lawyer cannot simply announce that a witness has turned hostile and proceed accordingly, and a witness does not become hostile by giving disappointing answers. Until the court makes the declaration, the ordinary position holds — including the rule that the party presenting a witness is not allowed to impeach that witness's credibility.
The three grounds, and how they differ
The rule lists three, and they are not interchangeable. Adverse interest is about where the witness's stake actually lies, which may be clear from their position rather than from anything they say. Unjustified reluctance to testify is about conduct on the stand, and note the qualifier — reluctance that is justified is not the ground. The third, having misled the party into calling him or her, addresses a witness who gave the calling party reason to expect one account and then gave another.
'Adequate showing' is a real requirement
The rule does not let the court declare hostility on impression alone; it requires an adequate showing of one of the three grounds. This is what keeps the mechanism from becoming a routine response to unhelpful testimony. A party who is merely dissatisfied with what its own witness has said has not shown any of the three things the rule names. The showing has to be directed at one of them specifically.
What follows the declaration
The consequence is substantial, which is why the threshold exists. Once declared, the unwilling or hostile witness may be impeached by the party presenting him in all respects as if he or she had been called by the adverse party — with one exception, that it may not be done by evidence of the witness's bad character. The witness may also be cross-examined by the adverse party, though that cross-examination must be confined to the subject matter of the examination-in-chief.