OpenSC is all about SmartCards?. SmartCards? are all about cryptography. Cryptography is something users don't care much about nor want to know about. At the same time - SmartCards? are usually tightly tied to the cardholder. So user interaction and UserInterface? are actually important components of the overall solutions that SmartCards? provide.
To sum up where exactly and how user interaction takes place, can take place or should take place, we need to know what layers and standards affect this area. Then we can find the most convinient and optimal path so that the whole usage of smartcards can be somewhat hidden and convenient for the user. To be more precise: user interaction is everything that the user _must_ do in normal cases - so user _has_ to authenticate to the card somehow, but she must not start other interactions - some application can have the initiative. Information to the end user (errors etc) falls into this category too.