It is well-known that one may not walk in front of somebody while he recites the Amida. Is it similarly forbidden to walk in front of somebody while he recites Kaddish?
The Hida (Rav Haim Yosef David Azulai, 1724-1806), in his Birkeh Yosef, references a manuscript written by Rav Yaakob Molcho (Jerusalem, 17th century) stating that it is forbidden to walk in front of somebody reciting Kaddish. This ruling is cited later approvingly by both the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) and Rav Yaakob Haim Sofer (Baghdad-Jerusalem, 1870-1939). The reason is that passing in front of the person can easily cause him to be distracted and lose concentration.
Hacham Ovadia Yosef clarified that this applies only through the recitation of "Da'amiran Be'alma Ve'imru Amen." The rest of Kaddish was added later, and is thus treated more leniently. Hence, it is permissible to walk in front of somebody while he recites these additions.
Unlike in the case of somebody reciting the Amida, it is permissible to sit within four Amot of somebody reciting Kaddish. The reason, as explained by Hacham Ovadia Yosef, is that one does not disrespect the recitation of Kaddish by sitting next to the person, since he responds to the Kaddish recitation. Halacha forbids sitting near a person reciting the Amida because this disrespects the recitation; in the case of Kaddish, however, one is actively participating by responding to the Kaddish, such that there is no display of disrespect, and so this is allowed.
Summary: It is forbidden to walk in front of somebody while he recites the main body of Kaddish – from the beginning until "Da'amiran Be'alma Ve'imru Amen." One is allowed to sit near somebody who is reciting Kaddish.