Lecturers generally hope that students will have read at least a good portion of the text because if they know nothing about it then they won’t understand a lecture on it, let alone be able to discuss it at a seminar. While it may be true that if you read a text after listening to a lecture you may read it with a greater level of understanding, you will understand the lecture better if you have read the text. The ideal is to read the text before the lecture and then look over your lecture notes later.
If you find yourself unable to read a text in advance it’s probably best to be honest about it but do please try to buy the book and take it to the seminar. I have had the unenviable task of dealing with a seminar room of 30 odd students, not one of whom had read the play or even bought it! The only copy in the room was mine. I did some hasty photocopies of the first scene and ‘we’ did a close analysis of it (ie I took them through it as I would read it). The students loved it but it did mean that the material I had prepared (in relation to the course objectives and assessments) couldn’t be covered.
I always went into seminars with plan A (most of the students have read a decent portion of the text and will have it with them), plan B (some of them will have read it and we’ll have enough copies to share) but having to come up with plan C (not one of them has even seen the book let alone bought and read it) on the spot was less than ideal.