At the entrance
Check clothing and bag rules before arriving. At St. Peter's the published rule is shoulders covered, no shorts above the knee, no miniskirts, and no hats inside, with security screening at the entrance. Put phones on silent and learn which spaces are reserved for prayer. If the church is celebrating a service, follow directions about where visitors may stand or move; St. Peter's closes to visitors entirely during papal ceremonies. A paid tour does not override those arrangements.
During the visit
Speak softly, keep explanations outside restricted prayer areas, and ask before photographing people. Some sites prohibit all photography in particular spaces: the Sistine Chapel bans both photography and talking. At Jasna Góra the icon is ceremonially unveiled and veiled at set times, and at the Holy Sepulchre six communities share the building under the 1852 Status Quo, so the schedule of one community governs access to a chapel. The absence of an obvious barrier is not permission to enter a sanctuary or other restricted area.
When participating in worship
Guests can attend respectfully without pretending to know every response. Communion has its own Catholic discipline, set out in the USCCB guidelines; ask a priest privately if unsure. Allow other pilgrims to pray without turning their devotion into a background for your photographs.