For people with mild symptoms you should do "watchful waiting" if the trauma happened less than 4 weeks ago.
Single episode "debriefing" after a traumatic incident should be avoided.
People should have access to early intervention trauma-focussed psychological therapy
https://www.nice.org.uk/guidance/cg26
(Although this guidance is going to be re-written soon).
First step is usually to index what the reactions are that make the trauma last. Those can go from violence, the will to fix it, over avoidance, to panic, and goes on quite a while. At an extremely high level you could perhaps say that you go over them, attempting to eliminate them one by one. But of course "fixing" panic reactions is done very differently (and much more carefully) from establishing the will to resolve the problem. Throwing in the deep end can work very well if the will to face the problem is the only problem. When we're talking about a situation that looks scary in movies but doesn't carry real risk (like non-venomous spiders) that will usually work, and in fact a lot of people will do just that without any help from a therapist. Best to verify beforehand though, as it certainly will make panic reactions worse. Panic is sometimes solved by role play, imagining, where possible, and planning out moves and counter moves before going anywhere near the real situation.
In a way a therapist only shares his own character with the patient. The theory is that this improves the patient, and the therapist is trained to experience minimal impact. It is advised though, that therapists pair up and have sessions with each other to verify that there is indeed only minimal impact, and fix if problems develop.
Throwing people in the deep end is usually unavoidable : people don't have the choice in the medium term, they have to face the real world, and therefore a good therapy should take that into account.