Rationalism and Constructivism
So my German friend was right, after all :) Every option has a pay-off, shaped only by the player's material interests. The option with the highest pay-off is the preferred option. Rationalists argue that the pay-off associated with each option, thus the preferences are fixed. Institutions they work within only constrain their means to reach given ends, and ideas are only language and symbols to justify their "self-interested policies." Agreement between two players is possible only if their interests coincide, or they accept each other's preferred options simultaneously through issue-linkage. Let's say Actor A advocates Policy A, and Actor B lobbies for Policy B. Actor A's pay-off associated with only Policy B is very low. But if Policy A and Policy B are passed simultaneously, the combined pay-offs may be acceptable to Actor A. Through issue linkage, pay-offs of Policy A and Policy B stay constant, but their combined pay-off also becomes relevant, breaking the stalemate.
Constructivists, on the other hand, argue that actors' understanding of the pay-offs may change, without an actual shift in material interests. Especially in crisis situations ("a policy window"), actors may be more open to new ideas - because they realize that what they always believed in doesn't work. A charismatic actor ("an institutional entrepreneur") comes up with a new idea, and uses such a policy window to persuade the other actors. As they interact and negotiate ("social learning" within the existing institutional framework,) actors change their minds about the pay-offs associated with each option.