How might transference and counter-transference issues manifest themselves in your work with participants? How about unconscious defenses?

Counter-transference becomes an issue when you project negative experiences you may have had into session with participant, and not understand what other person is feeling.

In a therapy context, transference refers to redirection of a client's feelings from a significant person to a therapist. Counter-transference is defined as redirection of a therapist's feelings toward a client, or more generally as a therapist's emotional entanglement with a client.
Almost all five self defense mechanism can get in the way if you don't know yourself. Possibly even regression if you were in very high stress situation.