For most people English is something the Parisians had to study in school, and thus seems a bit of a chore. People helping you out in English are making an extra effort, sometimes a considerable one. Younger people are much more likely to be fluent in English than older people.
Complicating things a bit more for visitors from North America is the fact that the French generally learn British English, as all in the European countries, in the "received pronunciation", aka "the Queen's English". Since most people in the US speak in this manner one will need to put effort into avoiding all slang and speaking clearly.
Likewise, the French taught in schools in English-speaking countries tends to be written French which is quite different from spoken French. Indeed, French spoken by native English speakers tends to be really hardly understandable by the French - do not be offended if people ask you to repeat, or seem not to understand you, they do not act out of snobbery. Keep your sense of humor, and if necessary, write down phrases or place names. And remember to speak slowly and clearly.
So, if it's your first time in France you will have some problems to understand what people are saying. They talk very fast, swallow some letters and make it all sound like beautiful music. So unless you have an advanced level and can at least sort of understand French Movies you should also assume that it will be difficult for people to understand what you are saying. Your French will grow when you have been a while in this amazing country and a glass of red wine will surely help the language to flow.
When in need of directions what you should do is this: find a younger person, or a person reading some book or magazine in English, who is obviously not in a hurry; say "hello" or "bonjour"; start by asking if the person speaks English (even if he/she's reading something in English), speak slowly and clearly; write down place names if necessary. Smile a lot. Also, carry a map; given the complexity of Paris streets it is difficult to explain how to find any particular address in any language, no matter how well you speak it.
What you should not do is this: stop a random person in the métro (like, say, some middle-aged hurried person who has a train to take), fail to greet them and say "where is place X or street Y". This will not go down well.
Now if you do speak French, remember two magic sentences : "Excusez-moi de vous déranger" and "Pourriez-vous m'aider?" —use them liberally - especially in shops; they will work wonders. However, in the parts of the city that tourists frequent the most (Tour Eiffel, Le Louvre, Champs-Elysées), the shopkeepers, information booths attendants, and other workers are likely to answer you in English, even if your French is advanced. These workers tend to deal with thousands of foreign-speaking tourists, and responding in English is often faster than repeating themselves in French. This is not the case in the rest of the city.
Tip: before you leave you may want to read a book like French or Foe by Polly Platt or Almost French by Sarah Turnbull — interesting, well written records from English speaking persons who live in France.
Tip: If you find yourself lost in the streets, a good idea is to find the nearest Hotel and ask the concierge for directions. Most speak English well-- but remember to be polite! A simple "Bonjour Monsieur, parlez-vous anglais?" should suffice.