I am not a Samaritans listener, however, I did apply and got accepted but declined the offer. The selection process for me was a 1 day information session and group interview. From that they would either invite you to a second, one to one interview or decline you. The 1-1 interview was not too difficult - 2 people on the panel and they asked you about 10 questions, mainly about your beliefs around suicide and how you would cope in the position. Also some questions around your life experience and what was the hardest thing in your life etc. Then they told you that day if they wanted you to train.
For training there was a quite strong commitment - you had to have 3 months free.