The Talmud (Bavli, B'rachos 18 amud 1) mentions a prohibition on wearing tzitzis or t'filin, or reading from a Torah scroll, in a cemetery, based on "one who taunts the pauper, that makes him a blasphemer" (Proverbs 17:5).
Based on this, the Shulchan Aruch forbids the following in a cemetery near a grave: wearing tzitzis exposed to view (OC 23, YD 367:4), wearing t'filin exposed to view (OC 45:1, YD 367:2), saying "Sh'ma" (OC 71:7, per Mishna B'rura :15), holding a Torah scroll (YD 282:4, per Shach :6), reciting from the Torah except in honor of the deceased (YD 367:3, Kaf Hachayim OC 23:1), and praying (YD 367:3) even kadish (Shach :3). (As always, consult your rabbi for practical guidance.)
Seemingly, the pauper-taunting is the fulfilling of a mitzva (command), alluding to the fact that he can still do so and the deceased cannot.
Is there a prohibition, then, on fulfilling other commands also, when apparent to onlookers, except in honor of the deceased? (If my father and I are near a grave and he asks me for his sweater that I'm holding, so that he may wear it, am I not allowed to pass it to him, because that would be obviously fulfilling the mitzva of honoring my father? If someone begs for charity near a grave, am I not allowed to give? If there's a fall hazard near a grave, am I forbidden from putting a railing up?) If (as I suspect) fulfilling all mitzvos is not prohibited, then why not? What's special about tzitzis, t'filin, reciting Torah and prayers, and holding a Torah scroll? And if fulfilling all mitzvos is in fact prohibited (when apparent to onlookers), why doesn't Shulchan Aruch say so as a blanket rule?