Requires your full attention. On them. A necessity for you to be a deep listener. To be friendly, compassionate as well as discerning.
And where appropriate, a giver of ideas.
Obvious. But why do we need reminding?
Think of the times you weren't. You dwelled on your own issues.
It's not for the first encounter. Unless they were a new found therapist. Or a rare connection when someone finally "got you" and offered up constructive help.
Be on your best form. If not - reschedule.