I'm looking for a noun that describes a person that experiences something while he is experiencing.
As an example (I look for the word for the general case), a young person (John) that accompanies a farmer at his work for few days, to check how is it feeling for him, in order to decide later if he wants to study agriculture. The missing word would be one word that describe John while he is in the farm:
John is a _____.
Another example: someone who is considering participating in a surfing camp, and takes a one-day trial to check whether it is for him. What I'm looking for is what to call him during that day.