In my experience of teaching students with ESL, I found that the more students communicated verbally using English skills versus reading esl books the more comfortable they became. This process requires a high level of trust with the cohorts, as well as the teacher. Building trust in the classroom can be challenging, but it is possible. Setting up boundaries within the classroom, such as respecting one another's efforts, providing positive feedback and avoiding situations that may cause a student to feel embarrassed by their efforts will be helpful. The use of one-on-one role playing that can be expanded to having the class set up various scenarios to present is also helpful. Examples of this process may be located in the esl books. The process of developing any skill is practice; learning to talk more in English is a skill and consistent practice is the key to improvement.
Watching movies and television shows in English can be beneficial. A lead character in a movie said that she learned to speak English, on her own, by watching English television shows. As all of the shows were British, she also developed a British accent.
Recently, I was in a fast-food shop and noticed that all the staff were speaking in Spanish. It was clear that they were struggling to talk more in English when trying to understand my order. The staff would probably benefit from speaking English while at work.
My thought is that students who remain quiet are experiencing some level of fear that they will be ridiculed or laughed at if their English skills are not perfect. Emphasizing the need to practice, as well as creating a safe place to practice are keys to improving their skills.