Patience and friendliness go a long way in Belize. Don't criticize local ways of doing things—there's usually a reason that may not be obvious to visitors—and, especially with officials, adopt a respectful attitude.
Belizeans generally are incredibly kind and friendly. Greet folks with a "Good morning" before asking for directions, for a table in a restaurant, or when entering a store or museum, for example. It will set a positive tone and you'll be received much more warmly for having done so.
Don't take pictures inside churches. Do not take pictures of indigenous people without first asking their permission. Offering them a small sum as thanks is customary.
With the exception of first-class buses based in Guatemala and Mexico, the seats on many buses often have three people seated abreast. Though tourists are often larger than the average local, you should respect the rule, and make room for others. It's fine to step into the aisle to let someone take a middle or window seat.
Business dress is casual. Men rarely wear suits and ties, and even the Belize prime minister appears at functions in a white shirt open at the neck.
English is Belize's official language. Spanish is widely spoken especially in northern and western Belize. Several Mayan dialects and the Garífuna language are also spoken. Some Mennonite communities speak a German dialect. Creole, or Kriol, which uses versions of English words and a West African–influenced grammar and syntax, is spoken as a first language by many Belizeans, especially around Belize City.
Around Tikal, wherever tourist traffic is heavy, you'll find a few English speakers; you'll have considerably less luck in places off the beaten path. In general, very little English is spoken in El Petén, and in some small villages in the region absolutely none. In addition, many Guatemalans will answer "yes" or "si" even if they don't understand your question, so as not to appear unkind or unhelpful. To minimize such confusion, try posing questions as "Where is so-and-so?" rather than asking "Is so-and-so this way?"