When you select cast to your production some of the creators has yellow background. That means they due to various reasons (mainly prestige but not only) they not want to cooperate with you. That is why they want higher salary then usual (this is "negotiation part"). This yellow part depends also on relation with given creator. (there are more things which is important but relation is taken into account) so it may happen that if you have lower prestige but good relation they will go from yellow to white rank. Still if they are in yellow zone if you got better relation the negotiated salary is smaller.
Other places where relations are taken into account (still here mainly positive/negative has impact)
"It seems that X may be interested in one of the motifs in the scenario 'Y'. Do you want to offer cooperation?"
After such question person still may reject cooperation even if motive is interested for him (but your relations are bad).
Other places
- Send role offer to person for cast in script
- Various recommendation of friends (or friends of friends) also checks the relations