This post is simply a list, to be added to, of research papers evidencing factors common to all types of counselling and psychotherapy which are cited as influencing treatment outcomes. Please feel free to add suggestions!
Cuijpers, P., Reijnders, M. & Huibers.M. (2019). The role of common factors in psychotherapy outcomes. Annual Review of Clinical Psychology, vol. 15 (publication date May 2019). Review in Advance first posted online on December 14, 2018. (Changes may still occur before final publication.)
Norcross & Wampold: discussion on Drug & Alcohol Findings (website).
Zuroff, D.C. & Blatt, S.J. (2006). The therapeutic relationship in the brief treatment of depression: contributions to clinical improvement and enhanced adaptive capacities. J Consult Clin Psychol. 74, 130-40.