So, when I write or speak a sentence with interrogative pronouns I need to put the interrogative pronoun in the same case it refers.
Renataf -- yes, it sounds like you more or less understand it now!

The only thing I would add is that in the example you gave ("What kind of fruit do you like?"), I'm pretty sure that most Russians nowadays would prefer to use какой instead of который -- thus, Какой фрукт ты любишь? Or as an example of the feminine accusative, Какую водку ты любишь? And in a seafood restaurant you could ask, Какая рыба у вас свежая сегодня? ("What kind of fish do you have fresh today?" -- note that какая, рыба, and свежая are all in the nominative, just as when one says У меня друг or У нас собака, etc.)

It's not that using который as an interrogative is grammatically wrong, but simply that it may sound a bit old-fashioned and uncolloquial in a question of this type, so in general it's better to use какой.

Of course, который is still used as a RELATIVE pronoun, as in:

Он поднял книгу, которая лежала на полу. ("He picked up the book, which was lying on the floor.")

But in today's Russian, который is not used so much as an INTERROGATIVE pronoun, except in some traditional fixed idioms.