As indie filmmakers you can't turn away any opportunity to market your film. In every process you go through to make the film you should give some consideration to what you can do make it more compelling and/or involve more people. Most films need actors and the process of finding these actors can also serve valid marketing functions.

More Actors, More Supporters

When you are putting together a project you might consider including lots of small roles for people. Instead of treating people as extras that are hauled in, shepherded to the set, and returned to pasture, make the extra parts special and cast for them. With some strategic placement of people throughout the film and careful choosing of who should fill those roles you will not only gain good extras, but people who will want to tell their friends about the movie. Actors can be great evangelists for your film because they want to be seen and in that process will tell others about your movie. You might even consider adding more small speaking parts so actors have even more incentive to tell others about the movie.

Generate Word Of Mouth By Casting Wide

Instead of treating casting as a burden that needs to get done ASAP, use it as a platform to spread word about the film. Don't only hold casting sessions. Deputize volunteer casting directors that go and recruit people that might be interesting to have in the film. The more people you meet, the more people will know about your movie, and the more likely you'll find the right fans for the movie as well as actors. Even if people don't actually want to be considered for casting it's likely something that they would mention to their friends. Your goal as marketer of the film is to give people a reason to talk to others about your movie.

Unique Audition Experience

Once you start casting your net wide to include all kinds of people you might make the audition process something that people will want to tell their friends about. Where is the casting session going to take place? Is there a fun process to find the place, like a treasure hunt of sorts? Is it a group casting session that is part social gathering? Rather than making it a grueling cattle call make it a cool get together with people that like what you are about. Meet and greet people and tell them about what you guys are planning, maybe videotape all the scenes you do in the audition and put them on YouTube. Or make a reality type documentary that you will put up online somewhere.

Lots of filmmaking tasks can be tweaked to serve the movie in other ways. Don't think just in terms of casting people that want to be in the movie, come up with more ways to talk to people and get them to talk to others where your movie is a part of the conversation and your marketing will be done for you.