Where can I find informations concerning client's willingness to express emotions during psychotherapy as the signal about his readiness to work on the solution of the problem which leads to behaviour change?
I would recommend to read some reviews on theoretical background of Motivational Interviewing.
Below some references concerning your question.
Vogel, D. L., & Wester, S. R. (2003). To seek help or not to seek help: The risks of self-disclosure. Journal of Counseling Psychology, 50, 351–361.
Vogel, D. L., & Wester, S. R., & Larson, L.M. (2003). Avoidance of Counseling: Psychological Factors That Inhibit Seeking Help. Journal of Counseling & Development, 85, 410-422.
Vogel, D. L., Wester, S. R., Wei, M., & Boysen, G. A. (2005). The role of outcome expectations and attitudes on decisions to seek professional help. Journal of Counseling Psychology, 52, 459–470.
Thanks for your very interesting question, Marta. I appreciated Beatrice's and Michael's excellent input. Along with motivational interviewing (MI) by Miller and Rollnick (2012), I'd recommend the Stages of Change (SOC) of The Transtheoretical Model (TTM) by Prochaska & DiClemente, (1983) and Prochaska, DiClemente, & Norcross (1992) for assessing the client's readiness and motivation to change.