Possible effects are, visually, as soon as people see a camera they tend to either pose, change body language, facial expressions, stop talking, shy away, cover their face and become self conscious or all of the above. One of the best places to photograph people is in crowds at festivals when people are too busy and too preoccupied to care, enjoying themselves, relaxed and happy. One method is to focus the camera first at the same distance in another direction, swing quickly, refocus fast and click. Sound is easier if the equipment is small but the problem in this instance is background noise. A zoom microphone can be useful if concealed.
Elderly people recounting their life experiences are excellent subjects.
They have lived through so much that the presence of recording equipment is inconsequential. They are delighted that anyone might be interested, they are enthusiastic to help young learners and essentially delighted that other nationals are making such efforts as to learn their language. For video recording, a video camera with a screen is excellent as the camera is pointed at a 90 degrees away from the observer and the object frequently is unaware that filming is taking place. This is particularly useful at festivals where the camera can be held up at arms length to catch the action with the screen tilted downwards. In a situation where the microphone was concealed at the front desk of a hotel, staff were aware of its presence, and pleased to cooperate.