Obviously every case turns on its own merits and the current human rights and security situation in the claimant's country of origin.
In short, the claimant's account must be consistent with the situation in his/her country of origin. You must go through the latest Country Packages and read the reports by Amnesty International, Human Rights Watch etc and pick out what is relevant to support the claimant's case but also what is against it.
Look at the country guidance case law - there may be cases similar to the circumstances described by the claimant.
The more evidence that a claimant can provide to corroborate his claim the better - newspaper articles, police reports, medical reports, court summons, political party membership cards, baptism certificates etc (depending on which of the five convention grounds the claim is based upon).
You must address the issue of internal relocation - can a claimant escape his persecutor by relocating to another part of his/her country of origin. This is particularly relevant where the persecutor is not the state but a certain warlord, family, criminal group, particular tribe/ethnic group.
Was state protection available to the claimant? Was the state unable or unwilling to protect the claimant. If the persecutor is the state itself or if there is a complete break down in law and order and government agencies in a state then state protection will not be available.
The claimant must be credible.