on to all their years of listening to comprehensible input.
You may know children of immigrant parents who can understand their parents' language quite well, but cannot speak it at all. Nancy Dorian (1981) noticed that although she had learned to speak Gaelic in the course of her research, young people with Gaelic speaking parents, although they could not speak the language at all, could often understand it better than she could. They had grown up with massive comprehensible input, and had developed a high degree of comprehension ability, but little or no speaking ability.
So it appears that massive comprehensible input can result in people having the ability to understand a language without necessarily being able to speak it well, or even to speak it at all. It appears that in order to learn to speak, you have to put a certain amount of effort into speaking. Somehow, I don't find that surprising.
You might ask whether it is possible to learn to speak without receiving much comprehensible input. Some linguists have told me that their speaking ability exceeded their comprehension ability, at least for a long time. Recently one told me that he could plan and execute a very complicated sentence in a certain African language, but that if he heard the same sentence in natural speech he would have difficulty understanding it. My experience in learning Blackfoot was similar. For a long time my speaking ability exceeded my comprehension ability. That is not a very good way to learn a language, for a number of reasons. For one thing, if you can say a lot more than you can understand, people will misjudge your general level of ability in the language, and speak to you in such a way that much of what they say will go over your head. This can make conversational experiences embarrassing and stressful, and discourage you from spending a lot of time conversing with people. So you want to keep the horse of comprehension ahead of the cart of speaking, while bearing in mind that comprehensible input in and of itself is not enough. You also need to speak.
In particular, it appears that you will need to put a reasonable amount of effort into conversational speech. Sure, you could concentrate on monologues, say by making long speeches to large audiences, but you might not know whether anyone understood you. By contrast, when you engage in one-on-one conversational interaction with people, it will often become obvious that you have failed to communicate, or have miscommunicated. What is more, the people with whom you converse will be able to help you to find ways to say what you are trying to say. It seems reasonable to think that this would contribute to your language learning.
Conversational interaction is an important source of comprehensible input as well. When you are involved in conversing with people, they will tend to adjust their speech to your level of ability. They may speak more slowly than normal, and use simpler vocabulary and simpler sentence structures, and repeat themselves a lot, and reword their sentences whenever you appear to be having trouble understanding them. Listen to yourself the next time you are talking to someone with limited English ability. You will probably find that you make these types of changes in your own speech in order to help them to understand you. Michael Long has demonstrated that the types of changes people make in their speech when they talk to foreigners really do make a significant difference in the ability of the foreigners to understand them (Long 1985).
Consider the nature of a conversation between a sympathetic native speaker and you as a language learner. You have a meaning you wish to express. You make a stab at it, but the person with whom you are conversing is either unsure of what you meant, or wonders whether you really meant what you appear to have meant. So she helps you to clarify yourself. Likewise, when she says something to you, it may go over your head, and so you get her to clarify her meaning. This back and forth process of a language learner and a sympathetic native speaker 