This rarely happens, but if it does you will receive a lot more money and be eligible to join the Screen Actors Guild . Many low budget films can not afford to hire experienced actors and will hold open calls. You can find auditions for these kinds of productions in trade publications like Backstage or even local newspapers. However, it's a good idea to be cautious when responding to these ads. Take a friend with you to the audition and know what you're getting into.
Most small films are wonderful experiences, but others can be purely exploitive.
Always use your best judgement. After building credits and experience doing some or all of the things listed above, you should be ready to find an agent. Unless you live in a major city, this may be difficult. But even some smaller cities have reputable agents who submit their clients for commercials and the occasional film. You may be the most talented actor on the planet, but you'll never get anywhere without determination.