It is the function of a witness to corroborate what has been alleged (T 543/95) and not to fill in the gaps in facts brought forward to support the case (T 374/02; confirmed more recently in e.g. T 1100/07, T 1028/11 and T 2054/11). It is necessary that a party who wishes to adduce evidence by means of a witness should indicate what factual details it wishes to prove by this means (T 374/02). The principle of free evaluation of evidence also applies to the hearing of witnesses under Art. 117(1)(d) EPC (T 482/89, OJ 1992, 646).

The EPC requires clear requests with regard to testimonies a party wishes witnesses to give, since the responsible department of the EPO must issue a decision regarding the taking of oral evidence (see Art. 117(1)(d) and R. 72(1) EPC). The board in T 374/02 was of the opinion that the "implicit offer of witnesses" did not specify what should be able to persuade the board to evaluate the evidence already existing in the file differently. Witnesses were meant to corroborate the facts, not to fill in gaps in the facts and arguments. The lack of an indication of the facts which were to be proven by testimonies of the three persons mentioned meant that the necessary conditions for the hearing of witnesses did not exist in the case before the board (other examples: T 2054/11; T 703/12, general reference to the party's submissions in opposition; T 1570/14, implicit request and requirements of Art. 12(2) RPBA 2007). T 1028/11, cited below, deals with a – justified – refusal to hear a witness and an alleged infringement of the right to be heard. In T 30/12, the board refused a request to rehear a witness (see below on the relevance of evidence offered). The boards in T 1363/14 and T 2238/15, ruling on alleged prior use, held that the opposition division should have ordered the hearing of the witnesses requested by the opponent to substantiate the facts set out in detail in the notice of opposition (see also T 1553/07 and chapter IV.C.2.2.8 d) "Alleged public prior use").