The answer to this question really depends on a few things: firstly, what sort of production your film will be; secondly, what age the actors you need should be; and most importantly, whether you are paying.
Unsurprisingly, the last point is normally the one which will have the most impact on your ability to get good actors for your production. If you can afford to pay, even just at union minimum levels, you’ll find that your options will be quite extensive. Start by putting together a brief for each character, including their playing age (normally a five year range, e.g. 25-30), gender, rough physical description, ethnicity (if relevant to the story). It’s also a good idea to include a 1-2 sentence dramatic description of who the character is, as this helps actors get a feel for whether the role might be something they would be interested in. Once you have your brief, get in touch with casting agencies in your local area and start the search.
If you don’t have enough money to pay the actors, all is not lost. Inevitably this will significantly narrow your options, but many actors may be willing to do you a favour if they like the script or are looking to boost their showreel. It’s likely you’ll have more success with younger actors, particularly those in, or just out of drama school. That said, it is well and truly possible to get older actors for nothing if you have a good script. Indeed, even big stars like Ian McKellen and John Hurt have been known to do shorts for free if they like the script (and are available at the time).
Regardless of whether you are paying your actors or not, the most important thing is to be honest about what the deal is when talking with potential cast members. This includes providing key information on things like: how much time they will be required for, shooting dates and locations, transportation arrangements between their house and the set, any special requirements for the shoot (e.g. action sequences, working at heights etc). You should also offer to pay all of the actors’ out of pocket expenses — important always, but essential if your actors are working for free. It’s also worth talking with them about the likely outlets for your film.
In terms of finding actors, you should try and cast your net as wide as possible to ensure you have the largest number of options to choose from. Areas such search include:
local and regional casting agencieslocal drama schoolslocal amateur theatre groupsThere are also a growing number of free (or very cheap) services on the web which help filmmakers locate actors. These include:
Star Now (UK) Mandy.com (UK)Constant Casting (UK) Shooting People (UK/US) Talent Circle (UK) Spotlight (UK)Fame 4 Anyone (UK/Worldwide)Feel free to suggest more services, particularly those in other countries.