One may recite Shema while standing, sitting, or lying down on one’s side (Berakhot 10a; SA 63:1). We learn from this rule that faith is not something detached from this world, something that can only be achieved under specific circumstances. Rather, the faith expressed in the recitation of Shema is a part of every life situation in this world, and so one may recite Shema in any situation.
Technically, one may recite Shema even while walking, as the Torah says, “when you walk on your way.” However, the Sages maintain that it is not proper for one to accept the yoke of heaven casually. Therefore, one who is walking should stand still while reciting first verse of Shema (SA 63:3; MB 9).
Because of the importance of the first paragraph, in which we accept the yoke of heaven, one must be careful not to occupy herself with anything else while saying it, nor gesture with her eyes, fingers, or lips (SA 63:6).
It is worth noting that mitzvot require kavana (SA 60:4). That is, in addition to the actual performance of the mitzva, one must also intend this action or speech-act to be a mitzva; just as one has a body and a soul and one cannot live without the other, so too, the mitzvot have both body and soul. The act is the body and the accompanying intent is the soul.