What is an interactive Grammar lesson?

Broadly speaking, interaction in English teaching can be defined in various ways. In fact, every single activity both inside and outside the lesson can be called a kind of interaction. Even a single handout can imply T's message to ss, and they answer T by solving the handout. Even a single eye contact between T and one student can be a significant interaction. Hence, I want to focus on my problem on interaction, that is, the balance of frequency and intensity of interaction.
Now I have a dilemma on interaction with students. If one teacher has only one student, just like a private teacher, my problem will never occur. Let me explain in detail. When I am in a class of 40 students, and interact with one of them, what do other 39 students do? The more intensively I interact with one student, the more bored other 39 students will become.
On the contrary, if I pursue too much the frequency of interaction, that is, how many students I can talk with in one lesson, the quality of each interaction will become worse. This is just a mechanical interaction, which I tend to fall on, if I focus on the "interaction" so much.
These extremes have shown us the clue to my question: the balance of frequency and intensity of interaction. Then, what is the best balance? Take Grammar lesson, for example. Suppose you have a grammar textbook. In the left page, there are sample sentences and some grammatical explanation. On the right page, there are about 20 questions, such as fill-in-the-blank types and paraphrasing, composition and so on. What can we interact with ss, using this textbook?
If T just explains to ss the sentences in the left page, and answers the exercises in the right page, which occurs in high schools very often, we can hardly call it an interactive lesson. That is because T doesn't need any Ss in order to keep this style. All T needs is the textbook and the knowledge. This is far from our ideal.
Then, how about asking the meaning of each sentence in the left page, and answers for questions in the right page? We also can hardly call it interactive. The reason is that the process of asking and answering tend to fall into the formal pattern or monotonous routine. What T looks at is just each student who is answering that question, and he does not care about the rest of other ss, which will surely worsen their motivation to study. I will call this situation a "Ritual Interaction", which I sometimes fell into before I started writing this journal.
My answer to these dilemmas so far is to use "the right method to the right place". For example, when I teach ss Comparative, which has much more items for mechanical memorization, I prepare the fill-in-the-blank handout about the left page and use it as a test of the lesson. If ss cannot get the good score, they must transcribe it as a penalty. The feeling that ss want to avoid the penalty can be a great motivation, at the same time, the penalty itself is a strong message from T that T wants ss to study that handout. This process cannot be called an interaction, at least visually, but I strongly believe that it is quite interactive because there is a lively interaction between T's intention and Ss' motivation. For grammatical topics which include many things to memorize, such as Comparatives, I believe that the intensive exercise with the handout after a quick explanation about how to memorize it, can be one of the most effective interactions.
Another example is how to introduce the continuative mode of relatives. Since the usage seems quite confusing to ss, the first thing they must do is to understand the function and usage, rather than to memorize them. The simple explanation, however, is far from the interaction. The first step to make the grammatical explanation more interactive is to draw ss' attention, for example, by using interesting examples as shown in "How many boyfriends does Hanako have?” T must look friendly and let ss consider the difference. Specifically, T must concern the eye contact, small talks with ss, poses, the blackboard, and so on. Each of these pieces in the explanation consists of good interaction, which is indispensable to improve ss' motivation.
In conclusion, interaction is an important tool to enhance ss' motivation to learn. T must use an appropriate interaction in each situation, which at the same time can avoid both the "ritual interaction" and the monotonous atmosphere of the one-sided explanation by T.