To offer an example, in a class of two, you first talk to a student one-on-one for a few minutes, then do the same with the second student. In so doing, you've established yourself as a participant rather than a guide. The students won't be as quick to volunteer information or participate in conversations unless you initiate and run them. This steals valuable talk time from the students, and creates more hesitant speakers inside and outside the classroom. Compare an activity in which you write three questions on the board, and instruct the students to pair up and sustain the conversation for at least five minutes. Always strive for an atmosphere in which the students take responsibility for the language they produce. I often use the following to measure my involvement in the class: If the students are in the middle of an activity when I write info on the board for the next step of the lesson, no one even notices until the activity begins to wind down.
Because the warm up opens the class session, it sets the atmosphere and expectations of the lesson. It also allows you important assessment opportunities, which will later determine the type of activities, who will partner with whom, and the scope of the lesson. Always give equal consideration to the warm up as to other steps of the lesson. The result will be a more focused and positive group of students performing to your expectations.