The role of a teacher in teaching languages to adults is not clear. By roles, I mean, for example: facilitator, manager, assessor, resources, participant and counselor.
Suppose learners concentrate on English (or another wide spread language) as second language where lots of learning materials are available and accessible, so the teacher is not the only available source of knowledge.
Also let's assume that we are talking about adult learners.
Are there studies that show what kinds of roles a teacher has in language acquisition?
I'd like to remind that I am interested in adult learners that already have life achievements, that are professional/qualified workers, graduated, able to process materials as part of their job duties (but not necessary related to linguistics) without nanny/nurse/tutor (like that is needed for children)...
I am looking for provable data based on research of groups, not for a personal or expert opinion.