Hostile witness

witness who testifies in a contrary manner to what is expected

In United States law, a hostile witness is a witness in a trial who testifies for the opposing party or a witness who offers adverse testimony to the calling party during direct examination. A witness called by the opposing party is presumed hostile. A witness called by the direct examiner can be declared hostile by a judge, at the request of the examiner, when the witness' testimony is openly antagonistic or clearly prejudiced to the opposing party. A party examining a hostile witness may question the witness as if in cross-examination, thus permitting the use of leading questions. A hostile witness is sometimes known as an adverse witness.
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Noun
1. a witness whose relationship to the opposing party is such that his or her testimony may be prejudiced against the opposing party; "a hostile witness can be asked leading questions and cross-examined"
(synonym) adverse witness
(hypernym) witness

During an examination-in-chief, a lawyer is not allowed to ask leading questions of their own witness. But, if that witness openly shows hostility against the interests (or the person) that the lawyer represents, the lawyer may ask the court to declare the witness 'hostile', after which, as an exception of the examination-in-chief rules, the lawyer may ask their own witness leading questions. - (read more on Hostile witness)