"The differences between British and American English involve not just spelling but also style, grammar, punctuation, typography and vocabulary as well as cultural" - Josephine Bacon
How true is that! Because of Word and Spellcheck we Brits needs to make sure that all our text has the correct spelling i.e. colour instead of color, behaviour instead of behavior throughout our documents.
Dialogue is noticeable because of the influx of American films (movies) - during the Second World War subtitles had to be used as we had never heard an American accent before and couldn't understand it.
Differences aren't confined to dialogue, and the most noticeable is the date - whereas we write dd/mm/yyyy the Americans write mm/dd/yyyy and no 'th' after the number. Full stops, as in 2.12.97 might not even be recognised as a date. Confusion can also arise as 1/12 in England is 1st December but in America it is 12 January!
Vocabulary-wise there are words used on both sides of the Atlantic that have different meanings, for example the pavement in England is the sidewalk in America. The pavement in America means the roadway or blacktop and there are many more: fieldstone (crazy paving); diaper (nappy); shade (blind); shades (sunglasses); dust ruffle (valance); valance (pelmet); faucet (tap) are just a few. The major differences occur in areas where the two cultures have diverged such as law, construction, architecture, banking and finance where for example a private ledger in the States is bought ledger in England.
Style and punctuation - such as can't, don't, etc - in England these abbreviations aren't used in advertising but Americans consider them too formal. Commas are more lavishly used in America as is the use of the single 'l' whereas we would use a double, i.e. dialled/dialed; travelled/traveled.
There are also regional variations in speech, however many Americans are amazed at our regional differences over what, for them, are such short distances.