The recruiter is the user in charge of the whole process. This user will be added to all the meetings with the applicant. Moreover is important to know that this user must belong to "Recruitment / Office Manage All applicants" or " Recruitment / Administrator " because of: 

1. Even if you add the user as the recruiter of that position it won't be allowed to access the recruitment app.  Or if the user has a “Recruitment / Interviewer” access it won't be able to see anything in the App.
2. The user could not modify and manage the Job Position.

The interviewer is the user in charge of interviewing the applicants in this case “The Interviewers set on the job position can see all Applicants in it. They have access to the information, the attachments, and the meeting management and can refuse him. You don't need to have Recruitment rights to be set as an interviewer”