Enthralled is the past form, and also the form we use like an adjective. If you are enthralled, something has caught you so fully that you keep watching or listening; you are not bored, and you are not half somewhere else. We usually say “enthralled by” a story, a film, a speaker. It is warmer and quieter than “obsessed,” and more complete than “interested.” You will meet it more often than the plain verb enthral, because English likes “I was enthralled” better than “it enthralled me,” though both are correct. The old “held as a slave” meaning is not what people mean in a cinema or a classroom.