(insofar as diplomats can be interesting, of course), is that many of their traits are 'self-perpetuating'. In other words, once a diplomat has aquired the first level of a trait, they will, over time, gain additional levels in that trait regardless of what you do with them.
These traits are: Talkative, Secretive, Religious Tolerance, Religious Intolerance. The first (and sometimes second) levels of these traits are all assigned at character creation, and will all increase over time.
The traits are actually 2 pairs of trait/antitraits. Talkative/Secretive and Religious Tolerance/Intolerance.
Talkative
20% chance on character creation
Every turn there is a 4% chance of an increase in the value
Value 2-3 : Influence -1, Bribery +5
Value 4-7 : Influence -2,
Value 8+ : Influence -3, Bribery -5
Secretive
20% chance on character creation
Every turn there is a 4% chance of an increase in the value
Value 2-3 : Influence +1
Value 4-7 : Influence +2, Bribery +5
Value 8+ : Influence +3, Bribery +10
Religious Tolerance
On creation, if the population in the region is <=70% your own religion, there is a 50% chance of aquiring this trait (plus a further 25% chance of it being at level 2). Every turn there is a 4% chance of an increase in the level.
Level 1 : Influence +1
Level 2 : Influence +2
Level 3 : Influence +3
Religious Intolerance
On creation, if the population in the region is >=80% your own religion, there is a 50% chance of aquiring this trait (plus a further 25% chance of it being at level 2). Every turn there is a 4% chance of an increase in the level.
Level 1 : Influence -1
Level 2 : Influence -2
Level 3 : Influence -3
Obviously having a high level of influence is a Good Thing if you want to negociate good terms for trade rights, map info, ceasefires, vassalage etc. And some of the above traits confer positive influence and others negative. A secretive diplomat with religious tolerance will eventually gain +6 influence, without you having to do a thing. Conversely, a talkative diplomat with religious intolerance will eventually have a -6 influence penalty to overcome, and there will be nothing you can do to stop that from happening.
So, the things to do are:
(1) Train your diplomats from regions with <=70% of your own religion, to try to obtain the religious tolerance trait series.
(2) Examine all diplomats immediately on creation - those with the talkative trait and/or religious intolerance need to be gotten rid of (try to arrange an 'accident' at sea), while those with secretive and tolerance are definite keepers - you can just sit them around doing nothing and they will increase in influence on their own!
Other things to do with your diplomats
Sit them in a city with a market (or greater) until they get the translator ancillary - this confers +2 influence (there is a 15% chance of getting this each turn).
As with other agent types - train them up. The Good/Bad Diplomat series confers up to +5 influence, so give them a bunch of easy missions such as trade rights, offering small gifts etc. 1/2/4/8/16 successes will give +1/+2/+3/+4/+5 influence. Note that failures will subtract from your success score, and enough of them will give you negative influence (-2/-4/-8 gives -1/-2/-3 influence). Avoid bribery missions for this reason.
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