There are no guidelines for the researchers to follow and so, they can approach the participants in any ethical manner to gain as much information as they possibly can for their research topic. The difference lies in the usage; obviously, the interview schedule is used by the interviewer during a face-to-face interaction, while the questionnaire is simply filled out by the respondent.
For example, where did the interview occur and when, was the respondent particularly nervous at any time? Were there any surprises during the interview? The rate and amount of responses are higher. It's usually easier for them to talk about the present and then work into the past or future.
If you decide to choose this interview method, understand that it offers a great deal of flexibility for you as a researcher. If he came prepared, then he can utilize that time properly.