As a comparison, consider the possible differences between a well-trained interpreter and someone who is not:
A trained Interpreter
An untrained bilingual speaker
Will pass on everything said, fully and accurately. Not just the main points, but every detail.
Is unlikely to pass on all the details accurately, causing significant distortion of the contents.
Will convey the way of speech and every subtlety, preserving feelings, e.g. reservation, doubt, objection, approval.
Tends to sanitise and make cosmetic changes, removing much useful information.
Will tell you and seek clarification if they don’t understand or miss something.
May bluff or drop any part of the message they don’t understand or find too technical or complicated.
Will take notes to ensure accuracy.
Tends to rely completely on short-term memory.
May interrupt to ask for clarification or repetition to ensure accuracy.
May sound very fluent because they don’t stop to clarify or check facts.
Is bound by a code of ethics to maintain confidentiality.
Not necessarily bound by a code of ethics, meaning they could pass on your private details to someone else.
Is aware of role boundaries requiring them to remain neutral. Will not give personal opinions or advice and will only do interpreting, no other tasks.
Is not aware of role boundaries, and may give personal opinions or advice, or perform other roles, such as providing transport.
Has had specific training in interpreting skills and for the sector in which they’re interpreting.
Will not have had any interpreting skills or sector-specific training.