Verbalise non-verbal things also The interpreter's medium of expression is purely verbal. He is supposed to render the meanings which the conversants may have expressed by a mixture of verbal and non-verbal communication by purely verbal means. Therfore he will use neat and sharp, nearly written-language sentence structures and refrain from using any avoidable intonations or interjections. Some extremely unprofessional interpreters such as those that frequently accompany Chinese delegations, even use this intrusion to gain the upper hand in conversation and displace their fellow delegation members. Written texts use more abstractions than colloquial ones.
Abstract notions are more open to interpretation.
An interpreter can't always know what the conversants mean by the concrete terms that at first appear taken out of life context. He will try to use more abstract terms instead and concretise them during the conversation, as he gradually gets to know the details of their meaning. This applies especially to technical terms. It is best if an interpreter can do some of this learning beforehand. But the short preparation time can be only well utilized by interpreters who by experience know what is important.