It is the function of a witness to corroborate what has been alleged and not to fill in the gaps in facts brought forward to support the case. For this reason, 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. In T 374/02, an "implicit offer of witnesses" that did not specify what might persuade the board to evaluate differently the evidence already existing in the file meant that the necessary conditions for the hearing of witnesses did not exist in the present case.
The EPC does not rule out that the parties to the proceedings offer as witnesses their employees (see e.g. T 482/89, OJ 1992, 646; T 124/88, T 830/90, OJ 1994, 713; T 443/93, T 937/93) or clients (see T 575/94). The allegation that a witness might be biased does not itself render the testimony inadmissible; suspicion of bias is rather a matter to be considered during the evaluation of evidence (T 838/92).
The boards of appeal have drawn a distinction between the hearing of witnesses and the hearing of opinions by experts: a witness is put forward to substantiate facts of which he/she has personal knowledge. In T 311/01 the appellant (opponent) offered witness testimony on the skilled person's knowledge and understanding of the cited prior art. However, the testimony was offered as evidence not of specific facts but of the knowledge and ideas of skilled persons in the technical field concerned, so that the appellant was in fact offering experts, not witnesses. Regarding itself expert enough with regard to the features and advantages described in the prior art, the board refused to hear the proposed "witnesses" (see also T 1511/06 and T 32/10).
Given that the composition of the boards of appeal includes at least two technically qualified members, experts are heard only in special circumstances, i.e. when a board does not consider itself in a position to decide upon a matter without further technical assistance. The assessment of patent law aspects on the basis of comprehensible technical facts, is a matter for the members of the board and not one for a technical expert (T 395/91). Moreover, if a board were to be active in seeking experts to help the case of a party, it might be open to accusations of partiality (T 375/00).