The rule I use is this: "If a word starts with a vowel sound, use "an". If it starts with a consonant sound, use "a". Therefore I would write "a hotel" and "an honest man", and also "a unicorn" and "an umbrella".
However, some other writers and especially older and historical writers do prefer to use "an" with a sounded "h" if the first syllable is unstressed, as in "hilarious"; or for words like "hotel" that have been relatively recently co-opted from the French language and where in the French the "h" was silent.
Of course, when our fictional characters speak, it may be that they do not use "correct grammar".
I'd say go with what feels right to you -- bearing in mind that your editor or publisher may hold contrary views. Be sure to read the house guidelines.
• By Etienne de L'Amour ~ Google+
